# Coordinate Geometry

1. Jan 11, 2007

### Harmony

The points P(cp,c/p) and Q(cq,c/q) lie on the rectangular hyperbola xy=c^2. Show that the equation of the chord PQ is pqy+x=c(p=q), and deduce the equation of the tangent at the point P.

I can do the proving. And I can find the tangent as well if the word "deduce" is not there. How can you deduce the equation by refering to the chord PQ?

2. Jan 11, 2007

### Gokul43201

Staff Emeritus
- post deleted...I was talking out of my hat -

Last edited: Jan 11, 2007
3. Jan 11, 2007

### HallsofIvy

You mean pqy+ x= c(p-q). Remember that we can think of a tangent, at P, as being the "limit" of the chords as q goes to p. Take the limit as q goes to p of pqy+ x= c(p- q).