1) а1 + 6d + a1 + 2d = -8
2a1+8d = -8
a1 +7d-a1-4d = -6
3d = -6
d = -2
2a1-16 = -8
2a1 = 8
a1 = 4
2) 6+12+18... -56
14*6 = 84
84+6 = 90 (2+ 78 = 90 и т.д.)
90* 1/2(14) = 90*7 = 630
630-56 = 574
ответ 574
3) d = -1.8-2.2 = -4
a3 = 2.2+ 4 = 6.2
a6 = -1.8 -4 = -5.8
a3+a6 = 0.4