(配方法)麻煩大家幫我睇下我有無計錯數。

x2- 9x + 9 = 0
x2- 9x +(9/2)^2 +9 = (9/2)^2
(x-9/2)^2 + 9 = 81/4
(x-9/2)^2 = 81/4 - 9 = 45/4
x-9/2 = rt(45)/2 or -rt(45)/2
x = 9/2+rt(45)/2 or 9/2-rt(45)/2
x = [9+3rt(5)] /2 or [9-3rt(5)]/2

I'm afraid you made a mistake in step 3 and 5.

x2- 9x + 9 = 0
x2- 9/2x +9/2 = 0 <----中間個9/2x 的 x 是在分數線中間的。
x2- 9/2x + 9/2 - 9/2 = -9/2 or x2- 9/2x = -9/2 <---------(should be -9/2 instead of 9/2)
x2- 9/2x +(-9/4)2=-9/2+(-9/4)2
(x-9/4)2=81/16-9/2 <---------------- (should be -9/2 + 81/16)
(x-9/4)2=9/16
x=9/4 ± 3/4
x= 12/4 or x = 6/4
x= 3 or x= 3/2

x^2- 9x + 9 = 0
x^2- 9/2x +9/2 = 0 < 呢行已經錯左, 點會- 9x 同 + 9都除2 而x^2就唔除2呢

試下x^2- 9x + 9 = 0
x^2- (18/2)x + 9 = 0
x^2- (18/2)x = -9
x^2- (18/2)x + 9^2 = -9 + 9^2
(x-9)^2 = -9 + 81
(x-9)^2 = 72
x-9 = 正負開方72
x = 9 + 開方72 or x = 9 - 開方72