數列和通項的答案!!

2 + 4 + ...+2n
= 2(1+2+...+n)
= 2(1+n)n/2
= n(n+1)

2.
設首項為a, 公差為d
2(2a +3d) = 24
5/2 (2a + 4d) - (2a +d) = 27

即 2a + 3d = 12 ..(1)
a + 3d = 9 ..(2)
(1) - (2) 得a = 3
所以 d = 2
通項 = 3 + (n-1) (2) = 1 + 2n