Determine whether or not the following claim is true for all regular expressions r1 and r2. The symbol ≡ stands for equivalence regular expressions in the sense that both expressions denote the same language.
(a) (r1*)* ≡ r1*
(b) r1*(r1 + r2)∗ ≡ (r1 + r2)∗
(c) (r1 + r2)∗ = (r1*r2*)∗
(d) (rs +r)*r = r(sr + r)*
All of the above true, I need proofs that I can understand and apply to similar questions.