等差数列an的前n项和为Sn,且a5+a21=a12,求S27

问题描述:

等差数列an的前n项和为Sn,且a5+a21=a12,求S27

由等差数列的规律:a5+a21 = 2a13 = a12因此知道:公差\x05d = a13 - a12 = -a13也就是\x05a12= -2d,\x05a13=d.而\x05S27 = 27(a1+a27)/2 =(27/2)(a1+a27) = (27/2)*(2a14) =27*a14又因为:\x05a14 = a13 + d = -d+d...