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July 27th, 2014, 03:45 PM   #1
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Question Completely Confused

I don't know how many of these things I can post in a day or even an hour. But this one has me completely confused.

-9x(5x + 4) + 10(x + 3)

Here is my calculation, which was completely wrong (still working with simplifying equations):
-9x(5x + 4) + 10(x + 3)
-45x + 36x + 10x + 30
19x + 30

Here was the answer the calculator gave me:

-9x(5x + 4) + 10(x + 3)

Reorder the terms:
-9x(4 + 5x) + 10(x + 3)
(4 * -9x + 5x * -9x) + 10(x + 3)
(-36x + -45x^2) + 10(x + 3)

Reorder the terms:
-36x + -45x^2 + 10(3 + x)
-36x + -45x^2 + (3 * 10 + x * 10)
-36x + -45x^2 + (30 + 10x)

Reorder the terms:
30 + -36x + 10x + -45x^2

Combine like terms: -36x + 10x = -26x
30 + -26x + -45x^2

Last edited by lwhitworth67; July 27th, 2014 at 04:00 PM. Reason: wrong calculation
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July 27th, 2014, 07:13 PM   #2
jks
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Hi lwhitworth67,

Quote:
Here is my calculation, which was completely wrong (still working with simplifying equations):
-9x(5x + 4) + 10(x + 3)
-45x + 36x + 10x + 30
The first two terms should be -45x^2 - 36x. I think you will find that this will lead to the given answer.
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July 27th, 2014, 10:48 PM   #3
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Quote:
Originally Posted by lwhitworth67 View Post

-9x(5x + 4) + 10(x + 3)
-45x + 36x + 10x + 30
19x + 30
$\displaystyle \ -9x(5x + 4) + 10(x + 3)=
\\\;\\
= -45x^2 - 36x + 10x + 30=
\\\;\\
= -45x^2 - 26x + 30 .
$
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