Query 4

course Mth 163

If your solution to stated problem does not match the given solution, you should self-critique per instructions atvvvv

http://vhcc2.vhcc.edu/dsmith/geninfo/labrynth_created_fall_05/levl1_22/levl2_81/file3_259.htm

.

Your solution, attempt at solution. If you are unable to attempt a solution, give a phrase-by-phrase interpretation of the problem along with a statement of

what you do or do not understand about it. This response should be given, based on the work you did in completing the assignment, before you look at the

given solution.

004. `query 4

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Question: `qWhere f(x) = x^3, what are f(-2), f(-a), f(x-4) and f(x) - 4?

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Your solution:

f(x) = x^3

f(-2), f(-a), f(x-4) and f(x) - 4

f(-2) = (-2)^3

f(-a) = (-a)^3

f(x-4) = (x-4)^3

f(x) - 4 = (x)^3 - 4

confidence rating: 3 OK OK

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Given Solution:

** COMMON ERROR WITH COMMENT: Where f(x) = x^3, f(-2) = -2^3

INSTRUCTOR CRITIQUE: Write that f(-2) = (-2)^3. By order of operations -2^3 = -(2^3) (you exponentiate first, before you apply the negative, which is

effectively a multiplication by -1). For an odd power like 3 it makes no difference in the end---in both cases the result is -8

COMMON ERROR WITH COMMENT: f(-a) = -a^3.

INSTRUCTOR COMMENT:f(-a) = (-a)^3, which because of the odd power is the same as -a^3. If g(x) = x^2, g(-a) would be (-a)^2 = a^2.

ANSWERS TO THE REMAINING TWO QUESTIONS:

f(x-4) = (x-4)^3. If you expand that you get (x^2 - 8 x + 16) ( x - 4) = x^3 - 12 x^4 + 48 x - 64.

In more detail the expansion is as follows:

(x-4)^3 = (x-4)(x-4)(x-4) = [ (x-4)(x-4) ] * (x-4)

= [ x ( x - 4) - 4 ( x - 4) ] ( x - 4)

= (x^2 - 4 x - 4 x + 16) ( x - 4)

= (x^2 - 8x + 16) ( x - 4)

= (x^2 - 8x + 16) * x - (x^2 - 8x + 16) * 4

= x^2 * x - 8 x * x + 16 * x - x^2 * 4 - (-8x) * 4 - 16 * 4

= x^3 - 8 x^2 + 16 x - 4 x^2 + 32 x - 64

= x^3 - 12 x^2 + 48 x - 64.

f(x) - 4 = x^3 - 4. **

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Self-critique (if necessary):

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Self-critique Rating:

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Question: `qWhere f(x) = 2^x, find f(2), f(-a), f(x+3) and f(x) + 3?

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Your solution:

f(x) = 2^x

f(2), f(-a), f(x+3) and f(x) + 3

f(2) = 2^(2) = 4

f(-a) = 2^(-a) = 1 / 2^a

f(x+3) = 2^(x+3)

f(x) + 3 = 2^x + 3

confidence rating: 3 OK OK ............ I had thought that I needed to go further, but did not know if I could, just making it too hard

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Given Solution:

** Where f(x) = 2^x we have:

f(2)= 2^2 or 4;

f(-a) = 2^(-a) = 1 / 2^a;

f(x+3) = 2^(x+3); and

f(x) + 3 = 2^x + 3. **

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Self-critique (if necessary):

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Question: `qquery functions given by meaningful names. What are some of the advantages of using meaningful names for functions?

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Your solution:

Meaningful names help me keep things sorted out in my head.

confidence rating: 3 OK OK

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Given Solution:

** TWO STUDENT RESPONSES:

Using the function notation gives more meaning to our equations. Example... 'depth(t) = ' is a lot more understandable than ' y = '

I read of the pros and cons in using names. I think using names helps to give meaning to the equation, especially if it's one you haven't looked at for a

couple days, you know right away what you were doing by reading the 'meaningful' notation.**

ONE MORE STUDENT RESPONSE:

It is much easier to keep track of what you are doing if you use meaningful names, particularly in multistep procedures.

When working with one three different functions, I could call them

f(x) = provides the value of the “x” coordinate for any given “y”

g(x) = original value of the data for the “x” coordinate for any given y

h(x) = the difference between the calculated value for x and the value for x in the data for any given value of y

Therefore: f(x) – g(x) = h(x)

True, but anything but easy to follow

However if I used the designations below, it would be much easier to keep track of what I was doing.

Graph(x) = provides the value of the “x” coordinate for any given “y”

Data(x) = original value of the data for the “x” coordinate for any given y

Resid(x) = the difference between the calculated value for x and the value for x in the data for any given value of y

Therefore: Graph(x) – Data(x) = Resid(x) True, and you know at a glance what it is trying to tell you.

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Self-critique (if necessary):

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Question: `qWhat were your results for value(0), value(2), value(t+3) and value(t+3)/value(t)?

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Your solution:

value(t) = $1000 (1.07)^t

value(0) = $1000 (1.07)^0 = $1000

value(2) = $1000 (1.07)^2

= $1000 * 1.1449

= $1,144.90

value(t+3) = $1000 (1.07)^(t + 3)

value(t+3)/value(t) = [$1000 (1.07)^(t + 3)] / [$1000 (1.07)^t]

= (1.07)^(t + 3) /(1.07)^t

= 1.07^t + 1.07^3 / 1.07^t

= 1.07^3

confidence rating: 3 OK OK

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Given Solution:

** Substitute very carefully and show your steps:

value(0) = $1000(1.07)^0 = $ 1000

value(2) = $1000(1.07)^2 = $1144.90

value(t + 3) = $1000(1.07)^(t + 3)

value(t + 3) / value (t) = [$1000(1.07)^(t + 3)] / [ $1000(1.07)^t] ,

which we simplify. The $1000 in the numerator can be divided by the $1000 in the denominator to give us

value(t+3) / value(t) = 1.07^(t+3) / [ 1.07^t].

By the laws of exponents 1.07^(t+3) = 1.07^t * 1.07^3 so we get

value(t+3) / value(t) = 1.07^t * 1.07^3 / [ 1.07^t].

The 1.07^t divides out and we end up with

value(t+3) / value(t) = 1.07^3. **

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Self-critique (if necessary):

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Question: `qWhat did you get for illumination(distance)/illumination(2*distance)? Show your work on this one.

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Your solution:

illumination(distance) = 50 / distance^2

illumination(distance)/illumination(2*distance)

illumination(distance)/illumination(2*distance) = [50 / distance^2] / [50 / (2 * distance)^2)]

= [50 / distance^2] * [(2 * distance)^2 / 50]

= (2 * distance)^2 / distance^2

= 4 * distance^2 / distance^2

= 4

confidence rating: 3 OK OK

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Given Solution:

** We substitute carefully and literally to get

illumination (distance) / illumination (2*distance) = [50 / distance^2] / [50 / (2*distance)^2]

which is a complex fraction and needs to be simplified. You invert the denominator and multiply to get

[ 50 / distance^2 ] * [ (2 * distance)^2 / 50 ] =

(2 * distance)^2 / distance^2 =

4 * distance^2 / distance^2

= 4. **

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Self-critique (if necessary):

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Question: `qquery #3. Sketch a reasonable graph of y = f(x), if it is known that f(2) = 80, f(5) = 40 and f(10) = 25. Explain how you constructed your

graph.

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Your solution:

Points are: (2,80) (5,40) (10,25)

I put the points on my graph paper and connected the dots

confidence rating: 3 OK OK

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Given Solution:

** STUDENT ANSWER WITH INSTRUCTOR COMMENT: I used graph paper counted each small square as 2 units in each direction, I put a dot for the points (2,80),

(5,40), & (10,25) and connected the dots with lines.

INSTRUCTOR COMMENT: The points could be connected with straight lines, but you might also have used a smooth curve. **

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Self-critique (if necessary):

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Question: `qwhat is your estimate of value of x for which f(x) = 60?

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Your solution:

2 + 5 = 7 / 2 = 3.5 for an average

So x= 3.5

confidence rating: 3 OK OK

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Given Solution:

**If your graph was linear from the point (2, 80) to the point (5, 40) you would estimate x = 3.5, since 3.5 is halfway from 2 to 5 and 60 is halfway from 80

to 40.

However with f(10) = 25 a straightforward smooth curve would be decreasing at a decreasing rate and the y = 60 value would probably occur just a bit earlier,

perhaps somewhere around x = 3.3**

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Self-critique (if necessary):

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Question: `qwhat is your estimate of the value f(7)?

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Your solution:

f(5) = 40

f(10) = 25

f(7) = [(40 + 25) / 2]

= 65/2

= 32.5

confidence rating: 3 OK ...... I got a good answer, but I'm not sure I came to it the right way?

your answer corresponds to the broken-line graph you described, and is perfectly OK

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Given Solution:

** STUDENT RESPONSE WITH INSTRUCTOR COMMENT: My estimation for the value of f(7) was f(7) = 34.

A linear graph between (5, 40) and (10, 25) would have slope -3 and would therefore decline by 6 units between x = 5 and x = 7, giving your estimate of 34.

However the graph is probably decreasing at a decreasing rate, implying a somewhat greater decline between 5 and 7 that would be implied by a straight-line

approximation.

A better estimate might be f(7) = 32 or 33. **

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Self-critique (if necessary):

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Question: `qwhat is your estimate of the difference between f(7) and f(9)?

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Your solution:

Difference is 5

confidence rating: 3 OK OK

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Given Solution:

** If f(7) = 32 and f(9) = 27, not unreasonable estimates, then the difference is 5. **

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Question: `qwhat is your estimate of the difference in x values between the points where f(x) = 70 and where f(x) = 30?

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Your solution:

I have 6

confidence rating: 3 OK OK

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Given Solution:

** f(x) will be 70 somewhere around x = 3 and f(x) will be 30 somewhere around x = 8 or 9. The difference in these x values is about 5 or 6.

On your graph you could draw horizontal lines at y = 70 and at y = 30. You could then project these lines down to the x axis. The distance between these

projection lines, which again should probably be around 5 or 6, can be estimated by the scale of the graph. **

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Self-critique (if necessary):

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Question: `qquery #4. temperature vs. clock time function y = temperature = T(t), what is the symbolic expression for each of the following: (Question above

was incomplete – assuming you were going to use the same examples from the f(x) nottions and the generalized modeling process.)

The temperature at time t = 3.

The temperature at time t = 5.

The change in temperature between t = 3 and t = 5.

The average of the temperatures at t = 3 and t = 5.

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Your solution:

y = temperature = T(t)

y = temperature = T(3)

y = temperature = T(5)

T(5) - T(3) =

[T(3) + T(5)] / 2 =

confidence rating:3 OK OK

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Given Solution:

** STUDENT SOLUTION WITH INSTRUCTOR COMMENT:

The temperature at time t = 3; T(3)The temperature at time t = 5; T(5)

The change in temperature between t = 3 and t = 5; T(3) - T(5)

The order of the expressions is important.

For example the change between 30 degrees and 80 degrees is 80 deg - 30 deg = 50 degrees; the change between 90 degrees and 20 degrees is 20 deg - 90 deg =

-70 deg.

The change between T(3) and T(5) is T(5) - T(3).

When we specify the change in a quantity we subtract the earlier value from the later. INSTRUCTOR COMMENT:

To average two numbers you add them and divide by 2.

The average of the temperatures at t = 3 and t = 5 is therefore

[T(3) + T(5)] /2 **

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Self-critique (if necessary):

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Question: `qWhat equation would we solve to find the clock time when the model predicts a temperature of 150? How would we find the length of time required

for the temperature to fall from 80 to 30?

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Your solution:

T(t) = 150

[T(t) = 30] - [T(t) = 80]

confidence rating: 3 OK OK

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Given Solution:

** GOOD STUDENT SOLUTION:

To find the clock time when the model predicts a temperature of 150 we would solve the equation T(t) = 150.

To find the length of time required for the temperature to fall from 80 to 30 we would first solve the equation T(t) = 80 to get the clock time at 80 degrees

then find the t value or clok time when T(t) = 30. Then we could subtract the smaller t value from the larger t value to get the answer.

[ value of t at T(t) = 30] - [ value of t at T(t) = 80)] **

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Question: `qquery. use the f(x) notation at every opportunity:For how long was the depth between 34 and 47 centimeters?

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Your solution:

f(t) = 34cm

f(t) = 47cm

confidence rating:2 .......... I got lost on this one, I think I half way understand the question?

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Given Solution:

** If depth = f(t) then you would solve f(t) = 34 to obtain t1 and f(t) = 47 to obtain t2. Then you would find abs(t2-t1).

We use the absolute value because we don't know which is greater, t1 or t2, and the questions was 'how long' rather than 'what is the change in clock time

... ' **

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Self-critique (if necessary):

you knew what to do; the only question is what sort of notation to use

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Question: `qBy how much did the depth change between t = 23 seconds and t = 34 seconds?

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Your solution:

t(34) - t(23)

f(t) = .028(t^2) - 2.38(t) + 92.68

f(34) = .028(34^2) - 2.38(34) + 92.68

f(34) = 32.36800 - 80.92 + 92.68 = 44.12800

f(t) = .028(t^2) - 2.38(t) + 92.68

f(23) = .028(23^2) - 2.38(23) + 92.68

f(23) = 14.81200 - 54.74 + 92.68 = 52.75200

44.12800 - 52.75200 = -8.62400

confidence rating: 3 OK OK, I went and used my model from assignment 2, right?

good

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Given Solution:

** This would be f(34) - f(23).

If for example if your model was f(t) = .01 t^2 - 1.5 t + 90, you would find that

f(34) = 50.6 and

f(23) = 60.8 so

f(34) - f(23) = 50.6 - 60.8 = -10.2. **

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Self-critique (if necessary):

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Question: `qOn the average, how many seconds did it take for the depth to change by 1 centimeter between t = 23 seconds and t = 34 seconds?

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Your solution:

f(34) - f(23)

11sec / [f(34) - f(23)]

f(t) = .01 t^2 - 1.5 t + 90

f(34) = .01 (34^2) - 1.5 (34) +90

f(34) = 11.56 - 51 + 90

f(34) = 50.56 cm

f(t) = .01 t^2 - 1.5 t + 90

f(23) = .01 (23^2) - 1.5 (23) + 90

f(23) = 60.8 cm

f(34) - f(23) = 50.6 cm - 60.8 cm = -10.2 cm

11 sec / [ f(34) - f(23) ] = 11 sec / (-10.2 cm) = -1.08 sec / cm

depth changes by 1 cm in 1.08 seconds

confidence rating: 3 OK OK

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Given Solution:

** Change in depth would be f(34) - f(23). Change in t would be 34 s - 23 s = 11 s. Thus change in t / change in depth = 11 s / [ f(34) - f(23) ]. If f(t)

gives depth in cm this result will be in seconds / centimeter, the ave number of seconds required for depth to change by 1 cm.

If for example if your model was f(t) = .01 t^2 - 1.5 t + 90, f(t) in cm, you would find that

f(34) = 50.6 cm and

f(23) = 60.8 cm so

f(34) - f(23) = 50.6 cm - 60.8 cm = -10.2 cm so that

11 s / [ f(34) - f(23) ] = 11 s / (-10.2 cm) = -1.08 sec / cm, indicating that depth decreases by 1 cm in 1.08 seconds. **

If your solution matches this one, you solved the problem as it was intended and did very well. However, after this solution had been in use for years, a

sharp-eyed student noticed that the problem is actually not well-posed. Consider the following:

VALID STUDENT OBJECTION (problem is actually not well-posed)

Solution shows decrease (indicating direction) rather than measure of change. Question would actually ask for an

absolute value – increase and or decrease would not be indicated?

INSTRUCTOR RESPONSE:

Very good.

Technically the question isn't well-posed. The change is the change and not the magnitude of the change, so we're asking for a positive change in depth. The

phrasing about the time interval is 'how long ... to change', which implies a positive time interval. The positive change doesn't occur in a positive time

interval; a negative time interval doesn't answer the question as phrased.

The question would have been well-posed had it asked 'How long does it take for the depth to decrease by 1 cm. ... etc.'.

I'm going to leave the phrasing of the question as is, and add this note to the solution. Most students who answer the question correctly will then have an

opportunity to consider the idea of a 'well-posed problem'.

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Self-critique (if necessary):

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Question: `qOn the average, how by many centimeters did the depth change per second between t = 23 seconds and t = 34 seconds?

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Your solution:

f(34) - f(23)

34 s - 23 s = 11 s

t = [ f(34) - f(23) ] / (11 s)

f(34) = 50.56 cm

f(23) = 60.8 cm

f(34) - f(23) = 50.6 cm - 60.8 cm = -10.2 cm

[ f(34) - f(23) ] / (11 s) = (-10.2 cm) / (11 s) = -.92 cm / sec

Between t = 23 sec and t = 34 sec depth decreases by -.92 cm per sec

confidence rating:3 OK OK

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Given Solution:

** Change in depth would be f(34) - f(23). Change in t would be 34 s - 23 s = 11 s. Thus change in depth / change in t = [ f(34) - f(23) ] / (11 s). If

f(t) gives depth in cm this result will be in centimeters / second, the ave number of centimeters of depth change during each second.

If for example if your model was f(t) = .01 t^2 - 1.5 t + 90, f(t) in cm, you would find that

f(34) = 50.6 cm and

f(23) = 60.8 cm so

f(34) - f(23) = 50.6 cm - 60.8 cm = -10.2 cm so that

[ f(34) - f(23) ] / (11 s) = (-10.2 cm) / (11 s) = -.92 cm / sec, indicating that between t = 23 sec and t = 34 sec depth decreases by -.92 cm in 1 sec, on

the average. **

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Self-critique (if necessary):

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Question: `qquery. A hypothetical depth vs. time model based on three points, none of which are actual data points. Describe how you constructed your

graph.

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Your solution:

I plotted my points and connected the dots

(0, 96) (30, 65) (70, 41)

confidence rating:

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Given Solution:

** STUDENT RESPONSE: I sketched a graph using the depth vs. time data:(0, 96), (10, 89), (20, 68), (30, 65), (40, 48), (50, 49), (60, 36), & (70, 41). **

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Question: `qWhat 3 data point did you use as a basis for your model?

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Your solution:

(56, 40) (31, 60) (18, 80)

confidence rating:

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Given Solution:

** STUDENT RESPONSE: After drawing a curved line through the scatter data I placed 3 dots on the graph at aproximately equal intervals. I obtained the x and

y locations form the axis, thier locations were at points (4,93), (24, 68) & (60, 41). I used these 3 data points as a basis for obtaining the model

equation.**

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Question: `qWhat was your function model?

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Your solution:

(56, 40) (31, 60) (18, 80)

depth = a(t^2) + b(t) + c

40 = a(56^2) + b(56) + c

60 = a(31^2) + b(31) + c

80 = a(18^2) + b(18) + c

40 = a(56^2) + b(56) + c

3136a + 56b + c = 40

60 = a(31^2) + b(31) + c

961a + 31b + c = 60

80 = a(18^2) + b(18) + c

324a + 18b + c = 80

3136a + 56b + c = 40

-961a - 31b - c = -60

2175a + 25b = -20

3136a + 56b + c = 40

-324a - 18b - c = -80

2812a + 38b = -40

2175a + 25b = -20

2812a + 38b = -40

38(2175a + 25b) = 38(-20)

-25(2812a + 38b) = -25(-40)

82650a = -760

-70300a = 1000

12350a = 240

a = 0.0194331984

2175(0.0194331984) + 25b = -20

42.2672065 + 25b = -20

25b = -20 - 42.2672065

25b = -62.267

b = - 2.49068

2812(0.0194331984) + 38(- 2.49) = -40

3136(0.0194331984) + 56(- 2.49068) + c = 40

60.9425102 - 139.47808 + c = 40

c = 40 + 78.5355698

c = 118.53557

961(0.0194331984) + 31(- 2.49068) + 118.53557 = 60

depth(t) = 0.01943t^2 - 2.49068t + 118.5356

confidence rating: 3 ok ok

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Given Solution:

** STUDENT RESPONSE CONTINUED: The function model obtained from points (4, 93), (24, 68), & (60, 41) is depth(t) = .0089t^2 - 1.4992t + 98.8544. **

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Self-critique (if necessary):

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Self-critique Rating:

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Question: `qWhat is the average deviation for your model?

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Your solution:

Deviation is 2.4816

confidence rating:3 OK OK

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Given Solution:

** STUDENT RESPONSE CONTINUED: I added a column 2 columns to the chart given in our assignment one labeled 'Model data' and the other 'Deviation'. I then

subtracted the model data readings from the corresponding given data to get the individual deviations. I then averaged out the numbers in the deviation

column. The average deviation turned out to be 3.880975.**

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Self-critique (if necessary):

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Self-critique Rating:

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Question: `qHow close is your model to the curve you sketched earlier?

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Your solution:

It is fairly close, but since the graph was averaged to begin with from the original data I figured that it would be.

confidence rating: 3 OK OK

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Given Solution:

** STUDENT RESPONSE CONTINUED: I was really suprised at how close the model curve matched the curve that I had sketched earlier. It was very close.**

Query Add comments on any surprises or insights you experienced as a result of this assignment.

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Your solution:

confidence rating:

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Given Solution:

** STUDENT RESPONSE: I was surprized that the scattered data points on the hypothetical depth vs. time model would fit a curve drawn out through it. The

thing that sank in for me with this was to not expect all data to be in neat patterns, but to look for a possible pattern in the data.

INSTRUCTOR COMMENT: Excellent observation **

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&#This looks good. See my notes. Let me know if you have any questions. &#