Question

用火柴棍拼图形,第一个图形用2根,第二个图形用7根,第3个图形用15根,问第N个图形用几根?

Answer 1

0 + 1*3 - 1 = 2

2 + 2*3 - 1 = 7

7 + 3*3 - 1 = 15

N= 3N(N+1)/2 - N