assignment 8

#$&*

course Mth 158

9/21 12

008. `*   8*********************************************

Question: *   R.8.12. Simplify the cube root of 54

 

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Your solution:

 

You can first go through all the perfect cube and see if 54 happens to be one of them. Since it isn't, you can use the greatest perfect cube that it can be divided by. In this case, that would be 27. So in your equation you have (27*2)^(1/3) and that can be simplified to. 3 * 2^(1/3)... and that's about it.

 

confidence rating #$&*: 3

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Given Solution:

* * The cube root of 54 is expressed as 54^(1/3).

 

The number 54 factors into 2 * 3 * 3 * 3, i.e., 2 * 3^3. Thus

 

54^(1/3) = (2 * 3^3) ^(1/3)

= 2^(1/3) * (3^3)^(1/3)

= 2^(1/3) * 3^(3 * 1/3)

= 2^(1/3) * 3^1

= 3 * 2^(1/3), i.e.,

3 * cube root of 2.

 

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Self-critique (if necessary): ok

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Self-critique Rating: ok

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Question: *   R.8.18. Simplify the cube root of (3 x y^2 / (81 x^4 y^2) ).

 

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Your solution:

Ok, first you can simplify what's being cubed- so (3xy^2) / (81 x^4 y^2) = 3/81x^3= 1/27x^3

and now you have (1/27x^3)^(1/3) this can be turned into 1/[(27x^3)^(1/3)] and since 27x^3 can be evenly cubed you get 1/(3x)

confidence rating #$&*: 3

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Given Solution:

The cube root of (3 x y^2 / (81 x^4 y^2) ) is

 

(3 x y^2 / (81 x^4 y^2) ) ^ (1/3) =

(1 / (27 x^3) ) ^(1/3) =

1 / ( (27)^(1/3) * ^x^3^(1/3) ) =

1 / ( (3^3)^(1/3) * (x^3)^(1/3) ) =

1 / ( 3^(3 * 1/3) * x^(3 * 1/3) ) =

1 / (3 * x) =

1 / (3x).

 

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Self-critique (if necessary): ok

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Self-critique Rating:ok

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Question: *   R.8.30. Simplify 2 sqrt(12) - 3 sqrt(27).

 

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Your solution:

Ok, so with 2 sqrt(12) you first try to square root 12, but the largest perfect square that 12 can be divided by is 4, so that part of the problem becomes 2 sqrt(4*3) or 2*2 sqrt(3) which is 4 sqrt(3)

same process with - 3 sqrt(27) it = -3 sqrt(9*3) = -3*3 sqrt(3) = -9 sqrt(3)

now the square root parts are the same so you can subtract 4 sqrt(3)-9 sqrt(3) and get -5 sqrt(3)

confidence rating #$&*: 3

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Given Solution:

2 sqrt(12) - 3 sqrt(27)

= 2 sqrt( 2*2*3) - 3 sqrt(3*3*3)

= 2 sqrt(2^2 * 3) - 3 sqrt(3^3)

= 2 sqrt(2^2) sqrt^3) - 3 sqrt(3^2) sqrt(3)

= (2 * 2 - 3 * 3) sqrt(3)

= (4 - 9) sqrt(3)

= -5 sqrt(3)

Extra Question: What is the simplified form of (2 sqrt(6) + 3) ( 3 sqrt(6)) and how did you get this result?

 

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Your solution:

(2 sqrt(6) + 3) ( 3 sqrt(6)) is what you start with. Multiply it by distributive law. So (2 sqrt(6)*( 3 sqrt(6)) and 3*( 3 sqrt(6)) is your problem. Rewrite the problem to (2*3) *(sqrt(6)(sqrt(6) + 9(sqrt6) and you get (2*3)=6, (sqrt(6)(sqrt(6)= 6 because they would have equaled sqrt(36) which is 6 and the problem is simplified to 6*6 + 9sqrt(6) , then 36+9sqrt(6)

confidence rating #$&*: 3

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Given Solution:

 

(2*sqrt(6) +3)(3*sqrt(6)) expands by the Distributive Law to give

 

(2*sqrt(6) * 3sqrt(6) + 3*3sqrt(6)), which we rewrite as

 

(2*3)(sqrt6*sqrt6) + 9 sqrt(6) =

 

(6*6) + 9sqrt(6) =

 

36 +9sqrt(6).

 

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Self-critique (if necessary): ok

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Self-critique Rating:ok

 

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Question: *   R.8. Expand (sqrt(x) + sqrt(5) )^2

 

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Your solution:

(sqrt(x) + sqrt(5) )^2= (sqrt(x) + sqrt(5))*(sqrt(x) + sqrt(5) distribute and get x+sqrt(5x) + sqrt(5x)+5

= x+2sqrt(5x) +5

confidence rating #$&*: 3

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Given Solution:

 

(sqrt(x) + sqrt(5) )^2

= (sqrt(x) + sqrt(5) ) * (sqrt(x) + sqrt(5) )

= sqrt(x) * (sqrt(x) + sqrt(5) ) + sqrt(5) * (sqrt(x) + sqrt(5) )

= sqrt(x) * sqrt(x) + sqrt(x) * sqrt(5) + sqrt(5) * sqrt(x) + sqrt(5) * sqrt(5)

= x + 2 sqrt(x) sqrt(5) + 5.

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Self-critique (if necessary):

????does 2 sqrt(x) sqrt(5) = 2sqrt(5x) or did I multiply that part wrong? I just figured that if you can multiply sqrt(6)*sqrt(6) and get sqrt(36)=6 then sqrt(x)*sqrt(5) would = sqrt(5x) ?????

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Self-critique Rating:ok

 

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Question:

*   R.8.42. What do you get when you rationalize the denominator of 3 / sqrt(2) and what steps did you follow to get this result?

 

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Your solution:

multiply [3/sqrt(2)]*[sqrt(2)/sqrt(2)] and get (3sqrt(2))/2

confidence rating #$&*: 3

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Given Solution:

Starting with 3/sqrt(2) we multiply numerator and denominator by sqrt(2) to get

 

(3*sqrt(2))/(sqrt(2)*sqrt(2)) =

 

(3 sqrt(2) ) /2.

 

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Self-critique (if necessary): ok

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Self-critique Rating:ok

 

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Question: *   R.8.48. Rationalize denominator of sqrt(2) / (sqrt(7) + 2)

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Your solution:

[sqrt(2)/(sqrt(7) + 2)]* [(sqrt(7)-2)/(sqrt(7)-2)] = [sqrt(2)*sqrt(7)-2]/[7-2sqrt(7)+2sqrt(7)-4]= [sqrt(2)*sqrt(7)-2]/(7-4) = [sqrt(2)*sqrt(7)-2]/3

confidence rating #$&*: 3

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

To rationalize the denominator sqrt(7) + 2 we multiply both numerator and denominator by sqrt(7) - 2.

We obtain

 

( sqrt(2) / (sqrt(7) + 2) ) * (sqrt(7) - 2) / (sqrt(7) - 2)

= sqrt(2) * (sqrt(7) - 2) / ( (sqrt(7) + 2) * ( sqrt(7) - 2) )

= sqrt(2) * (sqrt(7) - 2) / (sqrt(7) * sqrt(7) - 4)

= sqrt(2) * (sqrt(7) - 2 ) / (7 - 4)

= sqrt(2) * (sqrt(7) - 2 ) / 3.

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Self-critique (if necessary): ok

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Self-critique Rating:ok

 

Extra Question: What steps did you follow to simplify (x^3)^(1/6) and what is your result, assuming that x is positive and expressing your result with only positive exponents?

 

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Your solution:

umm... I guess written down on paper it can look different, but since say the sqrt(x) would = x^(1/2) because a^(n/m) you could say that in this equation (x^3)^(1/6), the 3=n and 6=m so you could make this x^(3/6) which would equal x^(1/2)

which is ironically sqrt(x)

confidence rating #$&*: 3

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Given Solution:

* *  Express radicals as exponents and use the laws of exponents.

 

(x^3)^(1/6) =

x^(3 * 1/6) =

x^(1/2). **

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Self-critique (if necessary):ok

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Self-critique Rating:ok

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Question: *   R.8.60. Simplify 25^(3/2).

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Your solution:

25^(3/2)= [sqrt(25)]^3 = 5^3= 125

confidence rating #$&*: 3

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

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Given Solution:

 

25^(3/2) =

(5^2)^(3/2) =

5^(2 * 3/2) =

5^(2 * 3/2) =

5^3.

 

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Self-critique (if necessary):ok

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Self-critique Rating:ok

 

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Question: *   R.8.72. Simplify and express with only positive exponents:

 

(xy)^(1/4) (x^2 y^2) ^(1/2) / (x^2 y)^(3/4).

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Your solution:

ok so this one is easier to work out if you leave the exponents as fractions...

start with [(xy)^(1/4)] *[(x^2 y^2)^(1/2)] / [(x^2 y)^(3/4)] (i'm separating the stuff by brackets for my own sanity)

for now the only thing you can do with [(xy)^(1/4)] is separate it into x^(1/4)*y^(1/4)

so just focusing on the fraction part for now we have [(x^2 y^2)^(1/2)] / [(x^2 y)^(3/4)]=

(x*y)/[(x^2 y)^(3/4)] =

(x*y)/[(x^2)^(3/4)* y^(3/4)]=

(x*y)/[x^(6/4) * y^(3/4)]=

(x*y)/[x^(3/2) * y^(3/4)]

now you can multiply the nominator(s) which is x^(1/4)*y^(1/4)*(x*y). Note! x=x^1(whole) so you can continue by saying x^(1/4)*x^1 *y^(1/4)*y^1 = x^(5/4)*y^(5/4)

and now your full equations reads [x^(5/4)*y^(5/4)]/[x^(3/2) * y^(3/4)]=

[x^(5/4)*y^(2/4)]/x^(3/2)=

[x^(5/4)*y^(1/2)]/x^(6/4)=

y^(1/2)/x^(1/4)

confidence rating #$&*: 3

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

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Given Solution:

 

(xy)^(1/4) (x^2 y^2) ^(1/2) / (x^2 y)^(3/4)

= x^(1/4) * y^(1/4) * (x^2)^(1/2) * y^2 ^ (1/2) / ( (x^2)^(3/4) * y^(3/4) )

= x^(1/4) * y^(1/4) * x^(2 * 1/2) * y^(2 * 1/2) / ( (x^(2 * 3/4) * y^(3/4) )

= x^(1/4) y^(1/4) * x^1 * y^1 / (x^(3/2) y^(3/4) )

= x^(1 + 1/4) y^(1 + 1/4) / (x^(3/2) y^(3/4) )

= x^(5/4) y^(5/4) / (x^(3/2) y^(3/4) )

= x^(5/4 - 3/2) y^(5/4 - 3/4)

= x^(5/4 - 6/4) y^(2/4)

= x^(-1/4) y^(1/2)

= y^(1/2) / x^(1/4).

 

STUDENT QUESTION

 

I wrote the entire given solution on paper to see how to solve, but I am still confused when it gets to the

= x^(1 + 1/4) y^(1 + 1/4) / (x^(3/2) y^(3/4)

How do you get 1 + ¼? Does the 1 come from the xy on the right of the numerator?

 

INSTRUCTOR RESPONSE

 

The numerator of the expression

x^(1/4) y^(1/4) * x^1 * y^1 / (x^(3/2) y^(3/4) )

contains two factors which are powers of x. The two are

x^(1/4) and x^1 (the latter could be written just as x, but to apply the laws of exponents it's not a bad idea to write the exponent explicitly).

When you multiply these two factors, the laws of exponent tell you that you get x^(1/4 + 1) = x^(5/4).

The same thing happens with the y factors.

 

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Self-critique (if necessary): ok

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Self-critique Rating:ok

 

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Question: *   R.8.84. Express with positive exponents:

 

( (9 - x^2) ^(1/2) + x^2 ( 9 - x^2) ^(-1/2) ) / (9 - x^2).

 

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Your Solution:

Here it goes...

[(9 - x^2) ^(1/2) + x^2 ( 9 - x^2) ^(-1/2) ]/ (9 - x^2) When dividing exponents you subtract them. So if you make this problem into two parts you can divide (9 - x^2) ^(1/2) from the numerator by the denominator and get (9-x^2)^(-1/2)

and you can divide ( 9 - x^2) ^(-1/2) by the same thing to get ( 9 - x^2) ^(-3/2) and so now you have

(9-x^2)^(-1/2) + x^2 ( 9 - x^2) ^(-3/2) to get rid of the negative signs in the exponents you have to turn these part into fractions so you get [1/(9-x^2)^(1/2)] + [x^2/ (9 - x^2) ^(3/2)]

confidence rating #$&*: 3

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

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Given Solution:

( (9 - x^2) ^(1/2) + x^2 ( 9 - x^2) ^(-1/2) ) / (9 - x^2) =

( (9 - x^2) ^(1/2) + x^2 / ( 9 - x^2) ^(1/2) ) / (9 - x^2) =

[ (9 - x^2) (1/2) / (9 - x^2)^1 ]   + [  x^2 / ( (9 - x^2)^(1/2) * (9 - x^2)^ 1) ] =

(9 - x^2) ^(-1/2) + x^2 / (9 - x^2)^(3/2) =

1 / (9 - x^2)^(1/2) + x^2 / (9 - x^2)^(3/2).

 

In the third step the exponent ^1 on the (9 - x^2) expressions wasn't necessary, but was included to explicitly show the exponents and the application of the laws of exponents.

 

The first term in the 4th step is obtained as follows:

 

(9 - x^2) (1/2) / (9 - x^2)^1 = (9 - x^2) ^ (1/2 - 1) = (9 - x^2)^(-1/2).

 

 

EXPANDED EXPLANATION OF STEPS

 

( (9 - x^2) ^(1/2) + x^2 ( 9 - x^2) ^(-1/2) ) / (9 - x^2) =

( (9 - x^2) ^(1/2) + x^2 / ( 9 - x^2) ^(1/2) ) / (9 - x^2)

In the above step we have replace (9 - x^2) ^ (-1/2) in the numerator by (9 - x^2)^(1/2) in the denominator, following the rule that a^-b = 1 / (a^b) with a = (9 - x^2) and b = 1/2.

 

( (9 - x^2) ^(1/2) + x^2 / ( 9 - x^2) ^(1/2) ) / (9 - x^2) =

[ (9 - x^2) (1/2) / (9 - x^2)^1 ]   + [  x^2 / ( (9 - x^2)^(1/2) * (9 - x^2)^ 1) ]

The above step is just the distributive law of multiplication over addition, in which we multiply through the expression ( (9 - x^2) ^(1/2) + x^2 / ( 9 - x^2) ^(1/2) ) by 1 / (9 - x^2).  The brackets [  ]  have been added to clarify the two terms in the resulting expression, but the expression has the same meaning without them.

 

[ (9 - x^2) (1/2) / (9 - x^2)^1 ]   + [  x^2 / ( (9 - x^2)^(1/2) * (9 - x^2)^ 1) ] =

(9 - x^2) ^(-1/2)   +   x^2 / (9 - x^2)^(3/2)

(9 - x^2) ^ (1/2) / (9 - x^2)^2 = (9 - x^2)^-1/2, by the laws of exponents; and (9 - x^2)^(1/2) * (9 - x^2) = (9 - x^2) ^(3/2) by the laws of exponents.

 

(9 - x^2) ^(-1/2) + x^2 / (9 - x^2)^(3/2) =

1 / (9 - x^2)^(1/2) + x^2 / (9 - x^2)^(3/2)

(9 - x^2) ^(-1/2) has been replaced by 1 /  (9 - x^2) ^(1/2), using a^-b = 1 / a^b.

 

All the exponents in the final expression are positive.

 

It would also be possible to factor out 1 / (9 - x^2)^(1/2), though this wasn't requested and isn't necessary in the problem as stated.  The result would be

 

1 / (9 - x^2)^(1/2) * ( 1 + x^2 / (9 - x^2) ).

 

This could be further simplified to

 

1 / (9 - x^2)^(1/2) * ( 9  / (9 - x^2) ) , which is equal to

9 / (9 - x^2)^(3/2)

 

You aren't expected to be able to read these expressions.  You are expected to be able to write them out in standard form; having done so you should understand.

 

However these expressions are fairly challenging, so some of the expressions will be depicted here

 

( (9 - x^2) ^(1/2) + x^2 ( 9 - x^2) ^(-1/2) ) / (9 - x^2) would be depicted in standard notation as

 

(9 - x^2) ^(-1/2) + x^2 / (9 - x^2)^(3/2) would be depicted in standard notation as

 

1 / (9 - x^2)^(1/2) + x^2 / (9 - x^2)^(3/2) would be depicted in standard notation as

 

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Self-critique (if necessary):

this one was pure torture, just so you know

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Self-critique Rating: ok

 

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Question: *   R.8.108. v = sqrt(64 h + v0^2); find v for init vel 0 height 4 ft; for init vel 0 and ht 16 ft; for init vel 4 ft / s and height 2 ft.

 

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Your Solution:

 

just plug in the #'s for v0 and height in the equation

Sqrt(64*4 +0)= sqrt(256)= 16

sqrt(64*16 +0)= sqrt(1024)= 32

sqrt(64*2 + 4^2)= sqrt(128 + 16)= sqrt(144)=12

confidence rating #$&*: 3

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Given Solution:

 

If initial velocity is 0 and height is 4 ft then we substitute v0 = 0 and h = 4 to obtain

 

v = sqrt(64 * 4 + 0^2) = sqrt(256) =16.

 

If initial velocity is 0 and height is 16 ft then we substitute v0 = 0 and h = 4 to obtain

 

v = sqrt(64 * 16 + 0^2) = sqrt(1024) = 32.

 

Note that 4 times the height results in only double the velocity.

 

If initial velocity is 4 ft / s and height is 2 ft then we substitute v0 = 4 and h = 2 to obtain

 

v = sqrt(64 * 2 + 4^2) = sqrt(144) =12.

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Self-critique (if necessary): ok

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Self-critique Rating: ok

Extra Question: What is the simplified form of (24)^(1/3) and how did you get this result?

 

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Your solution:

 

find the largest perfect cube that 24 can be divided by and you get 24^(1/3)= (8*3)^(1/3)= 2* 3^(1/3)

confidence rating #$&*: 3

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

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Given Solution:

 

* *  (24)^(1/3) =

 

(8 * 3)^(1/3) =

 

8^(1/3) * 3^(1/3) =

 

2 * 3^(1/3) **

 

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Self-critique (if necessary):ok

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Self-critique Rating:ok

 

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Question:

 

Extra Question: What is the simplified form of (x^2 y)^(1/3) * (125 x^3)^(1/3) / ( 8 x^3 y^4)^(1/3) and how did you get this result?

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Your solution:

 

(x^2 y)^(1/3) * (125 x^3)^(1/3) / ( 8 x^3 y^4)^(1/3)

simplifying the second part first- (125 x^3)^(1/3) / ( 8 x^3 y^4)^(1/3)= 5x/2xy(y)^(1/3)=5/2y(y)^(1/3)

now rearrange the first part (x^2 y)^(1/3) to get x^(2/3)*y^(1/3)

put the 2 parts back together- [x^(2/3)*y^(1/3)]/[5/2y(y)^(1/3)]=

[5(x^(2/3)]/(2y)

confidence rating #$&*: 3

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

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Given Solution:

* *  (x^2y)^(1/3) * (125x^3)^(1/3)/ ( 8 x^3y^4)^(1/3)

 

(x^(2/3)y^(1/3)* (5x)/[ 8^(1/3) * xy(y^(1/3)]

 

(x^(2/3)(5x) / ( 2 xy)

 

5( x^(5/3)) / ( 2 xy)

 

5x(x^(2/3)) / ( 2 xy)

 

5 ( x^(2/3) ) / (2 y) **

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Self-critique (if necessary): ok

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Self-critique Rating: ok

 

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Question:  Extra question. What is the simplified form of sqrt( 4 ( x+4)^2 ) and how did you get this result?

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Your solution:

sqrt( 4 ( x+4)^2 )= 2|x+4|

because sqrt of 4=2 and sqrt of (x+4)^2= (x+4) except not because whether x is a positive or negative# it's going to become positive once it's squared so it has to be |x+4|

confidence rating #$&*: 3

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

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Given Solution:

 

We use two ideas in this solution:

 

sqrt(a b) = sqrt(a) * sqrt(b) and

sqrt(x^2) = | x |

 

To understand why sqrt(x^2) = | x | and not just x consider the following:

 

Let x = 5.  Then sqrt(x^2) = sqrt( 5^2 ) = sqrt(25) = 5, so sqrt(x^2) = x.

It is also clear that in this case, | x | = | 5 | = 5, so | x | = x, and we can say that sqrt(x^2) = | x |.

Now let x = -5.  We get sqrt(x^2) =  sqrt( (-5)^2 ) = sqrt(25) = 5. 

 

In this case sqrt(x^2) = 5 but x is not equal to 5, so sqrt(x^2) is not x. 

 

However, in this case | x | = | -5 | = 5, so it is the case the sqrt(x^2) = | x |.

 

Using these ideas we get

 

sqrt( 4 ( x+4)^2 ) = sqrt(4) * sqrt( (x+4)^2 ) = 2 * | x+4 | **

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Self-critique (if necessary): ok

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Self-critique Rating:ok

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

?????Did you feel sorry for us and make the last bit easier after that one nasty question ;] ????"

Self-critique (if necessary):

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Self-critique rating:

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

?????Did you feel sorry for us and make the last bit easier after that one nasty question ;] ????"

@& You handled the torture well, though that wasn't my intent. The torture part, that is.*@

&#This looks very good. Let me know if you have any questions. &#