Find the standard form of the equation of the circle with endpoints of a diameter at the points

Hello : 
let two points : A (a,b) , B (a' , b')
the center of the circle of diameter :  [AB] is (X0 , Y0) : 
X0 = (a+b)/2    and   Y0 = (a'+b')/2
the ridus is : r = AB/2
AB = √((a'-a)²+(b'-b)²)
the standard form of the equation of the circle is : 
(x-X0)² + (y -Y0)² = (AB/2)²

I’m a smart person so that’s why i think it was 6