Pythagoras theorem is a^2 + b^2 = c^2 and a circle has an equation x^2 + y^2 = a^2 .Is there a relation between a right angle triangle and a circle?

Yes they're related! A circle is the locus of a point, which is always equidistant from the center.

Now $P$ is a point on the circle of radius $r$ with co-ordinates $(x,y)$.

By Pythagoras theorem

$r^2=x^2+y^2$

Which is the equation of the circle!