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
Análisis de problemas de secuencia aritmética
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