let X belongs to A
X subset of A
XCA=>X belongs to P(A)
=>X belongs P(B) (...P(A)=P(B) )
=>X subset of B=>X belongs to B
X belongs to A =>X belongs to B . .. A subset of B (1)
similarly through the same steps,
Y belongs to B=>Y belongs to A ...B subset of A(2)
From (1)&(2)A=B