Question

a1=2,an=87,Sn=801. Найти d и n.

Answer 1

an=a1+(n-1)d=2+(n-1)d=87,

Sn=(2a1+(n-1)d)n/2=(4+(n-1)d)n/2=801,

(n-1)d=85,

(4+85)n=801*2,

89n=1602,

n=18,

d=85/(18-1)=5