Math 163
Your 'question form' report has been received. Scroll down through the document to see any comments I might have inserted, and my final comment at the end.
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I am having trouble with the equations. I am adding when I need to subtract or vice versa.
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I think I know why I am messing up on these problems. I went back to the CD and looked at the class notes again and worked through them step by step with the instructions to see if my answers were coming out to be the same and I am messing up when eliminating b. Here is a step by step explanation of what I’m doing
The following was copied from the CD and pasted here:
• We choose to eliminate b.
• To do so we multiply the first equation by 80 and the second by -39, so that the coefficients of b are equal and opposite.
• Then the sum of the two equations gives us a single equation, with b eliminated and only a remaining as a parameter.
121 a + 11 b + c = 80
2500 a + 50 b + c = 50
169000 a + 130 b + c = 10
?? Up to here I understand perfectly??
(2) – (1): 2500 a + 50 b + c = 50
- (121 a + 11 b + c = 80)
result: 2379 a + 39 b = - 30
(3) – (2): 14400 a + 80 b = -40
?? Here I get a little confused. I remember asking you this before and I seemed to answer my own question, but I have somehow forgotten how I solved it. It seems to me if you subtract (3) – (2) it would be:
169000 a + 130 b + c = 10
- 2500 a + 50 b + c = 50
giving you the result of: 166500 a + 80 b = -40 but the answer says 14400 a + 80 b = -40
I don’t know what I’m doing wrong.??
The handwritten pictures and explanations in the video files are correct.
However the 'typed-out' pictures in that Class Notes file have errors. Instead of 169 000 the coefficient of a in the third equation is 16 900. (It appears that in one picture the 1 is also left off the number, so it reads 6900 or 69000).
I'm sorry that error has caused you so much confusion. You are doing everything correctly.
I've made the correction in the Class Notes as posted on the webpage; I can't correct the DVD, but this should straighten you out.
Next step is
2379 a + 39 b = -30
14400 a + 80 b = -40
??This step I understand. You would multiply
80 (2379 a + 39 b) = 80 ( -30)
-39 (14400 a + 80 b) = -39 (-40)
Then you get into the sum of the two equations and I seem to get messed up here.
190320 a + 3120 b = -2400
-561600 a -33120 b = 1560 (Here I get 1560 and the solution is showing – 1560 Is the – sign showing us to subtract instead of adding?)
I don’t know why this is confusing me. It all seems so simple, but I know if I don’t figure this out early in the semester it will just keep confusing me??
Always remember that errors on my end are possible. There aren't that many errors in the Precalculus I course (and there are now far fewer in the Mth 158, thanks to your attention to detail). However there were errors in two of the 'typed-out' figures used in the Class Notes.