Consider the sum (a) Write the sum in sigma notation. (b) Use the identities from question 3 to prove that for n > 2 n(n+2)+(n+1)(n+3) + (n +2)(n+4)+...+3n(3n+2). n(n+ 2) + (n + 1)(n+3) + (n + 2)(n+4)+...+3n(3n+2) 13n(2n +3n+1) 3