Question

A9=3×A3
При делении A7 на A3=2 и остаток 1
Найти S10

Answer 1

a1+8d=3(a1+2d)⇒a1+8d=3a1+6d⇒2a1=2d⇒a1=d
a1+6d=2(a1+2d)+1⇒a1+6d=2a1+4d+1
a1+6a1=2a1+4a1+1
7a1-6a1=1
a1=1
d=1
S10=(2a1+9d)*10/2
S10=(2+9)*5=55