3+6+9+…+66+69求等差数列的和怎么算

Question

如题所述

Answer 1

等差数列前n项和公式，等差数列通项公式，求出该等差数列的项数。

Answer 2

3+6+9+.......+66+69
=(3+69)X[(69-3)÷3+1]÷2
=72X23÷2
=36X23
=36X20+36X3
=720+108
=828