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

a1+8d=3(a1+2d)⇒a1+8d=3a1+6d⇒2a1=2d⇒a1=da1+6d=2(a1+2d)+1⇒a1+6d=2a1+4d+1a1+6a1=2a1+4a1+17a1-6a1=1a1=1d=1S10=(2a1+9d)*10/2S10=(2+9)*5=55