5. Instead of using induction, we can sometimes use previously proven results about a summation to obtain results about a different summation. (a) Use the result in Progress Check 4.3 to prove the following proposition: 3n (n + 1) For each natural number n, 3+6+9+...+ 3n 2 (b) Subtractn from each side of the equation in Part (a). On the left side of this equation, explain why this can be done by subtracting 1 from each term in the summation. (c) Algebraically simplify the right side of the equation in Part (b) to obtain a formula for the sum 2 + 5+ 8 + ... + (3n - 1). Compare this to Exercise (3a)