According to figure – it is clear that centre will be ( h , h ) radius will be equal to h.
( x – h )2 + ( y – h )2 = h2
x2 + y2 — 2.h.x — 2.k.y + h2 = 0
But the centre of the circle could be any of the four quadrants. Then co-ordinates of the centre taken as
( ± h , ± h ).
Then equation of the circle is —
x2 + y2 ± 2.h.x ± 2.k.y + h2 = 0