Theorem (proof that)

a) A Delta B = AÚ B if A Subscription B= Null

b) A Subscription (B Ù C) = (A Subscription B) Ù (A Subscription C)

sorry for bad typing , I got error. better form : http://mathhelpforum.com/discrete-math/226661-theorem-proof.html#post812654

please help me.

thank you.