數一題! help!

(n-2)*180-100 /n = 360/n
(n-2)*18 - 10/n = 36/n
(n-2)*9 - 5/n =18/n
9n - 18 - 5/n = 18/n
9n*(n-2) - 5 = 18
9n*(n-2) = 23
n(n-2) = 23/9
n^2 - 2n = 23/9 -> n^2 = n square = n * n
n^2 - 2n + 1 = 23/9 + 1
(n-1)(n-1) = 32/9
(n-1)^2 = 32/9
n-1 = +/- (sqrt(32/9)) -> sqrt = square root
n = +/- (sqrt(32/9)) + 1

therefore:
n = 2.8856 or -0.8856

一係你打錯問題, 一係答案錯,
你試吓 放 n=9 入條式, 兩邊答案都不同, 計不到的~