x = 0 , y = 0 and the line x/a +y/b = 1
By these lines we can say the vertices of the triangle are
O(0,0) A(a,0) and B(0,b)
By seeing them we can say that the triangle is right angled triangle with a right anle at the origin.
So its hypotenuse is AB.
The circumcenter of a right angled triangle will be always the mid point of its hypotenuse.
So, circumcenter = Mid point of AB =(a+0/2, 0+b/2)=(a/2,b/2)