a1=2,an=87,Sn=801. Найти d и n.
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
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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