# Homework Help: Calc Question: Tangent Lines

1. Oct 24, 2005

### joshb110

I'm royally confused... dont even know where to start..

Find the point(s) on the graph of y = 1/x where the tangent line is parallel to 4x + 9y = 3. The m of 4x + 9y = 3 is -4/9 and sadly, thats all i know...

2. Oct 24, 2005

### joshb110

3. Oct 24, 2005

### amcavoy

First of all, what is the derivative of 1/x. Do you know how to calculate this? Remember that the derivative at some point x0 is the slope of the graph at that point.

4. Oct 24, 2005

### joshb110

hi. the derivative of 1/x = -1/ x^2, i believe.

5. Oct 24, 2005

### joshb110

what do i do next? do i just graph the two equations?

6. Oct 24, 2005

### HallsofIvy

You mean as opposed to actually answering the question??

The problem, as you posted it, "Find the point(s) on the graph of y = 1/x where the tangent line is parallel to 4x + 9y = 3."

You know that the slope of 4x+ 9y= 3 is -4/9, you know that the derivative of y= 1/x at any x is y'= -1/x2 and you know, because apmcavoy just told you, that the derivative at a point is the slope of the tangent line there. For what x is the derivative equal to -4/9?