Question

аn=21, n=7, Sn=105. Найдите а₁ и d.

Answer 1

$S_n=frac{a_1+a_n}{2}*n\105=frac{a_1+21}{2}*7\a_1+21=frac{105*2}{7}\a_1+21=30\a_1=9\\a_n=a_1+(n-1)d\21=9+(7-1)d\d=frac{21-9}{6}\d=2$