✔ 最佳答案
1) Let x rabbbits be he bought , then original cost = $ 1600/x for each rabbit. The man sold $ 1600/x + 40 for each. After 5 rabbits have died , there are x - 5 remaining rabbits. (x - 5)(1600/x + 40) = 1600 + 200 (x - 5)(1600 + 40x) = 1800x (x - 5)(40 + x) = 45x x^2 + 40x - 5x - 200 = 45x x^2 - 10x - 200 = 0 (x - 20)(x + 10) = 0 x = 20 or x = - 10(rejected)The man bought 20 rabbits.2)1/N + 1/(N+2) = 1 / [(1/6)(N+6)]2(N+1) / [N(N+2)] = 6 / (N+6)3N(N+2) = (N+1)(N+6)3N^2 + 6N = N^2 + 7N + 62N^2 - N - 6 = 0(2N + 3)(N - 2) = 0N = - 3/2 (rejected)
or
N = 23)27x^2 + 12 = 09x^2 + 4 = 0(3x)^2 = - 4x = ± (√- 4)/3x = ± (√4)√(-1)/3x = ± 2i / 3
