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...