Posted by **NaNa** on Thursday, October 11, 2007 at 1:09am.

Use the definition of the derivative to find f'(x)

1. f(x) 2x^2 + x - 1

Before the final step, my 'result' looked weird and I think I did something wrong in my arithmetic...someone correct me?

Here's my work...

BEGINNING STEP:

lim [2(x + deltaX)^2 - (x + deltaX) - (2x^2 + x - 1)] / deltaX

near the end step:

lim [4x + 2deltaX - 2x^2 - 1] / deltaX

Thanks

- Calculus - need help! -
**Quidditch**, Thursday, October 11, 2007 at 1:41am
You started correctly, but you must have a math error in the intermediate steps.

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

=((2x2+4(x*deltaX) + 2deltaX^2 + x + deltaX - 1) - (2x^2 + x - 1) ) /deltaX

=(4(x*deltaX) + 2*deltaX^2 + deltaX)/deltaX

=4x + deltaX + 1

limit as delatX goes to 0

=4x + 1

- Calculus - need help! -
**Quidditch**, Thursday, October 11, 2007 at 2:05am
Ooops!

the step before the limit statement should have read:

4x + 2deltaX + 1

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