# Equation of a line that is tangent to f(x)

1. Sep 21, 2008

### Squiller

In order to find the equation of a line that is tangent to f(x) and goes through point P on f, you gotta find the derivative of f(x) at P, but how would you go about solving a problem where you have to find the equation of a line tangent to f(x) that goes through point P, but P is NOT on the graph of f.

2. Sep 21, 2008

Re: Tangents

Just to get my facts straight: you have a function $$f(x)$$, a point $$P$$ that is not on the graph of $$f(x)$$, and you want an equation of a line that

1. passes through the point $$P$$, and
2. is tangent to the graph of $$f(x)$$

If this is correct, what else is stated in the problem - do you have a specific function $$f$$, at what point(s) is the line to be tangent, etc. Further, what have you tried?

3. Sep 21, 2008

### Squiller

Re: Tangents

f(x) = 4x-x2

Question: Find the equations of the lines that pass through P(2,7) and are tangent to the graph of f(x).

(P is not on f(x).)
Thats all the problem states.

Ive tried finding f'(x) and plugging f' into the Line equation y=mx+b.

y=(4-2x)x+b.

Then plugging in Point P.

7=(4-2x)2+b - Im not really sure if this is heading in the right direction.