Hey everyone, I'm a little stuck on this problem -- any help would be very much appreciated.

i) Prove that if a quadrilateral is inscribed in a circle, then the sum of its oppsoite angles is 180 degrees.

(in other words, prove: m<A + m<C = m<B + m<D = 180 degrees)

and part two of the question is -- What kinds of parallelograms can be inscribed in circles?

I'm thinking that it is ALL parallelograms but it would be nice to verify that. Thanks for any help