Question

Sn是等差数列｛an｝的前n项和，已知a2=3,a6=11则S7等于几

选项A 13 B 35 C 49 D 63
答案C
不明白 希望有详细的过程

Answer 1

a2+a6=a1+a7
S7=(a1+a7)*7/2=(a2+a6)*7/2=49
这里有个关系是
Am+Am=Ap+Aq 条件为m+n=p+q

Answer 2

a6 = a2 + 4d
11 = 3 + 4d
d = 2
a1 = a2 - d = 1
a7 = a6 + d = 13
S7 = (1 + 13)×7÷2 = 49

所以选 C