更新1: 这是一个 等比数列
上面那个，不要乱答！！！ ar-a=35 a(r-1)=3 r-1=3/a ar^3+560=ar^2 ar^3-ar^2=-560 ar^2(r-1)=-560 代入
(ar^2)(3-a)=-560 3ar^2-a^2r^2=-560 3ar^2+560=(ar)^2 3r+560=0 3r=-560 r=-560/3
错呀jacky仔！你识唔识factorization架！
ar – 35 = a …(1) ar^3 + 560 = ar^2…(2) (1): a = 35/ (r – 1) …(3) Put (3) into (2)
[ 35/ (r – 1) ] r^3 + 560 = [35/ (r – 1)] r^2 [35r^3 / (r – 1) ] – [ 35r^2 / (r – 1) ] = -560 35r^3 – 35r^2 = -560r + 560 35r^3 – 35r^2 + 560r – 560 = 0 5r^3 – 5r^2 + 80r – 80 = 0 抽”7” r^3 – r^2 + 16r – 16 = 0抽”5” (r – 1)(r -4) (r + 4) = 0 r = 1 (rej) OR r = 4 OR r = -4
ar-35=a ar^3+560=ar^2 (ar^3+560)*0=(ar^2)*0 0=0
