# Homework Help: Find derivative of f(a) for f(t)=(2t+1)/(t+3) using the definition of a derivative

1. Oct 11, 2012

### Al3x L3g3nd

1. The problem statement, all variables and given/known data

Find derivative of f(a) for f(t)=(2t+1)/(t+3) using the definition of a derivative

2. Relevant equations

f '(a)=lim as x goes to a of (f(x)-f(a))/(x-a)

3. The attempt at a solution

f '(a)=lim as x goes to a of (f(x)-f(a))/(x-a)
f '(a)=lim as t goes to a of (((2t+1)/(t+3))-((2a+1)/(a+3)))/(t-a)
f '(a)=lim as t goes to a of (((2t+1)(a+3))/((t+3)(a+3)))-(((2a+1)(t+3))/((a+3)(t+3)))
simplified and got
f '(a)=lim as t goes to a of ((-5a+5t)/((a+3)(t+3)))/(t-a)

I don't know where to go from here.

Last edited: Oct 11, 2012
2. Oct 11, 2012

### LCKurtz

Re: Find derivative of f(a) for f(t)=(2t+1)/(t+3) using the definition of a derivativ

Rewriting your last equation in readable form:$$\frac{-5a+5t}{(a+3)(t+3)(t-a)}$$You are almost there. Factor a 5 out of the numerator, cancel like factors, and let $t\to a$ and you will have it.

3. Oct 11, 2012

### Al3x L3g3nd

Re: Find derivative of f(a) for f(t)=(2t+1)/(t+3) using the definition of a derivativ

wow i feel dumb for not realizing that.

thanks :)