Differentiation wrong answer Watch

srose
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#1
Report Thread starter 10 years ago
#1
Hi
Here is the question I am stuck on:

Find the derivative of each of the following:

(x-2)(x+2)

I did this:

f(x) = x^2 + x -2x - 2
f(x) = x^2 - x - 2
f'(x) = 2x

but the answer is 2x - 1. What am I doing wrong?

Thanks
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Adjective
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#2
Report 10 years ago
#2
I assume there's a typo in your first expression, since (x+2)(x-2) is not equal to  x^2 - x - 2 .

Check you have differentiated each term of  x^2 - x - 2 .
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boy
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#3
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The -x is the same as -1x.
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srose
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#4
Report Thread starter 10 years ago
#4
Hi
yeh, that was a typo. I have figured that question out now.

I have another problem now:

1/2x +5x^3 at x = -1

I got the answer 19 but it is 16.
Here is my working:

f(x) = 2x^-2 + 5x
f(x) = -4x^-3 + 15x^2
f(-1) = 4+15
f(-1) = 19
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