Hello AlvinCYGradient PQ =

Gradient QR =

PQRS is a square

(since ) (1)

Now also satisfies , using (d), and again using

Subtract this equation from (1):

or, if not, , by dividing through by the common factor.

Now . Hence for real values of . So .

Hence and so, eliminating in equation (1):

I seem to have proved this in the reverse order, so I suspect there may be another method. However, I think the above reasoning is quite sound!

Grandad