а8 =25
а3 = a8 -5d
а13 = a8 +5d
а3+а13 = (a8 -5d) +(a8 +5d) = 2*a8 = 2*25 = 50
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Решение смотри во вложениях.
4) (1,1d - c^3)(1,21d^2 + 1,1 c^3 * d + c^6) - 1,331d^3 + 2c^9 = (1,331d^3 + 1,21c^3*d^2 + 1,1c^6 * d - 1,21c^3*d^2 - 1,1c^6 * d - c^9) = c^9.