(a)The product rule (also called the chain rule)states that:P(X1,X2,,Xn)=P(X1)P(X2|X1)P(X3|X1,X2)..P(Xn|X1,X2,,Xn-1)Now,prove the above rule in reverse direction as stated below:P(X1)P(X2 I X1)P(X3 | X1,X2)...P(Xn|X1,X2,...Xn-1)=P(X1,X2,...,Xn)(b)Prove that zeDomain()P(A=B=b)=1,where Domain(A)represents all thevalues that the random variable A can take,and the random variable B is assigned toits value b.