解方程 求r

上面那個，不要亂答！！！

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

錯呀jacky仔！你識唔識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