# Homework Help: Need help with basic dervivative problem

1. Dec 29, 2011

### TheKracken

1. The problem statement, all variables and given/known data
they want f'(x)
of f(x)=2x^2 +x-1

2. Relevant equations
I know how to use the power rule and such and get that answer to be 4x+1...but I am practicing using the definition of a dervivative and I keep getting 4x-1
...so ya need help

relevent equations: f'(x)= Lim h→0 of f(x+h) - f(x) all over h

3. The attempt at a solution
started out with
2(x+h)^2 +(x+h) -(2x^2 +x-1) all over h

then I got 2x^2 +2h^2 +2xh +x+h-1-2x^2 - x +1 all over h

and cancled out and got
2h^2 +4xh +h -1 all over h

then I factored out the h and got
h(2h+4x)-1 all over h

cancled out the h and got
2h+4x-1 all over 1

entered in 0 for h and i got 4x-1
so what the heck did I do wrong??? haha, I think I had my set up wrong from the beginning??? anyone willing to help me out here, just started self studying calculus so a little confused....

2. Dec 29, 2011

### eumyang

This is wrong. It should be
$$\frac{2(x + h)^2 + (x + h) \textbf{ - 1} - (2x^2 + x - 1)}{h}$$
This isn't right either. This should be
$$\frac{2x^2 + \textbf{4xh} + 2h^2 + x + h - 1 - 2x^2 - x + 1}{h}$$
Nope. This should be
$$\frac{4xh + 2h^2 + h}{h}$$
You should be able to figure out the rest from here.

Last edited: Dec 29, 2011
3. Dec 29, 2011

### TheKracken

right i got it....wrong set up, thank you very much :) now I see how to set those up haha, thank you.