А10 = а1 + 9d
будем искать а1 и d
а1 +d + a1 +5d = 14, 2a1 +6d = 14, a1 +3d = 7, a1 = 7 - 3d
(a1 +2d)(a1 +7d) = 76, a1^2 +9a1d +14d^2 = 76, (7 -3d)^2 +9d(7 -3d) +14d^2 = 76,
49 - 42d + 9d^2 + 63d - 27d^2 +14d^2 = 76
-4d^2 + 21d - 27 = 0
4d^2 -21d +27 = 0
D = 9
d1 = (21 +3)/8= 3
d2 = (21 -3)/8 = 18/8 = 9/4 = 2,25
1) а1 = 7 - 3d= 7 - 3*3= -2
2) a1 = 7 - 3d = 7 - 3*2,25 = 0,25( не подходит по условию задачи)
а10 = а1 + 9d = -2 +9*3 =-25