Hello UltraGirl Quote:

Originally Posted by

**UltraGirl** Hello

P is a variable point on xy=c^2. The tangent at P cuts the x axis at A and y axis at B. Q is the fourth vertex of OAQB. Show that the locus of Q is xy = 4c^2.

Ummm could someone please explain to me how on earth do we find Q. The question does not provide any information as to what OAQB is, so how exactly do you know what to do to find Q?

I've drawn the diagram and my guess is that Q's also on the hyperbola...but I don't know how to find it's point :(

As *Debsta *has said, if we assume that OAQB is a rectangle, then, from my answer to your first post, the coordinates of Q are

$\displaystyle \Big(2ct, \frac{2c}{t}\Big)$

So if Q is represented by the general point $\displaystyle (x,y)$, we have:

$\displaystyle x = 2ct, y = \frac{2c}{t}$

i.e.

$\displaystyle xy = 4c^2$

Grandad