(20%) (a) (15%) Prove that the number of the distinct solutions to the equation x1+x2++xn=k,xi=0,1,2,,k,i is (n+k1k)= (20%) (a) (15%) Prove that the number of the distinct solutions to the equation x1+x2++xn=k,xi=0,1,2,,k,i is (n+k1k)=