consider x being Element of V such that

A2: x in A by A1, SUBSET_1:4;

take x ; :: thesis: x is Element of A

thus x is Element of A by A2; :: thesis: verum

A2: x in A by A1, SUBSET_1:4;

take x ; :: thesis: x is Element of A

thus x is Element of A by A2; :: thesis: verum