# Thread: Given Derivative Thru a Point

1. ## Given Derivative Thru a Point

Another question..

f'(x)=1/XSQRED + 2X and goes through (-1,1).

I need to find a function that has that derivative and passes through that point

2. Originally Posted by Super Mallow
Another question..

f'(x)=1/XSQRED + 2X and goes through (-1,1).

I need to find a function that has that derivative and passes through that point
we want $f(x) = \int f'(x)~dx$ and $f(-1) = 1$ as an initial condition. who do you think you should proceed?

3. Hello, Super Mallow!

$f'(x)\:=\:\frac{1}{x^2} + 2x$ and goes through (-1,1).

Find a function that has that derivative and passes through that point.

We have: . $f'(x) \;=\;x^{-2} + 2x$

Integrate: . $f(x) \;=\;-x^{-1} + x^2 + C \;=\;-\frac{1}{x} + x^2 + C$

We are told that, when $x = -1,\:f(\text{-}1) = 1$

. . So we have: . $-\frac{1}{\text{-}1} + (\text{-}1)^2 + C \;=\;1\quad\Rightarrow\quad C \:=\:-1$

Therefore: . $f(x)\;=\;-\frac{1}{x} + x^2 - 1$

4. I forgot about C =)

That explains it, thank you!