I'm having a problem with the following question. If anyone can help, that would be very much appreciated.

(1) Suppose that (a,b) is on the circle (x^2)+(y^2)=(r^2). Show that the line ax+by=(r^2) is tangent to the circle at (a,b).

PLEASE, IF ANYONE CAN HELP, I WOULD LOVE IT.

Thank you in advance