Question

If A=B prove AUB=AnB

Answer 1

If two sets are same let a and bthen we can say a= b
according to definition of intersection it is function that contains common enements from both setsas a=b so all elemnts are commonso a= a n bor b = a n b
in union the function has all elements of both sets and in set no element repeats so 
a u b = aor a u b = b
as a u b  = a or b and a n b = a or b so we can conclude here that
a u b = a n b as a = b // defined above