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