A2=8; A10=40 Найти S12

Question

A2=8; A10=40
Найти S12

Answer 1

D=(a10-a2)/(10-2)=(40-8)/8=4
a1=a2-d=8-4=4
a12=a10+2d=40+8=48
S12=(a1+a12)*12/2=(4+48)*6=52*6=312