Подстановка условия
[src](((a - 7)*(a + 7))/(a - 7))*(a + 7) при a = -1
(((a - 7)*(a + 7))/(a - 7))*(a + 7)
a−7(a−7)(a+7)(a+7) ((((-1) - 7)*((-1) + 7))/((-1) - 7))*((-1) + 7)
(−1)−7((−1)−7)((−1)+7)((−1)+7) (((-1 - 7)*(-1 + 7))/(-1 - 7))*(-1 + 7)
−7−1(−7−1)(−1+7)(−1+7)