Ответ: X4=x2+2d70= — 10+2d2d=80d=40×1=x2-d= — 10-40= — 50S9=(x1+x9)/2*9X9=x1+8d= — 50+8*40=270S9=( — 50+270)/2*9=220/2*9=110*9=990