Calculus  need help!
posted by NaNa on .
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

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 
Ooops!
the step before the limit statement should have read:
4x + 2deltaX + 1