等差数列问题分析
S3 = 3a 1+3 * 2d/2 = 3a 1+3d
s 11 = 11a 1+11 * 10d/2 = 11a 1+55d
S3 = s 11 ∴3a1+3d=11a1+55d ∴d=-2
an = a 1+(n-1)d = 13-2(n-1)≥0 ∴n=7
S7 = 7a 1+7 * 6 *(2)/2 = 7 * 13-7 * 6 = 49
s 11 = 11a 1+11 * 10d/2 = 11a 1+55d
S3 = s 11 ∴3a1+3d=11a1+55d ∴d=-2
an = a 1+(n-1)d = 13-2(n-1)≥0 ∴n=7
S7 = 7a 1+7 * 6 *(2)/2 = 7 * 13-7 * 6 = 49