Mario bought 90 feet of rope for $21.60. He needs 60 more feet. If the unit price is the same, how much will he pay for the extra 60 feet of rope?

Use mathematical induction to prove the statement is true for all positive integers n, or show why it is false. 1^2 + 4^2 + 7^2 + + (3n - 2)^2 = [n(6n^2-3n-1)/2]