Re: [求助] 高一數學 數與座標系
※ 引述《kkboy123 (KKBOY)》之銘言：
: 4.題目：
: x,y為實數 |x|<1 |(x-y)/(1-xy)|<1 求y的範圍
|x| < 1 => x^2 < 1
|(x-y)/(1-xy)| < 1 => (x-y)^2 / (1-xy)^2 < 1
=> x^2 - 2xy + y^2 < 1 - 2xy + (xy)^2
=> (xy)^2 - x^2 - y^2 + 1 > 0
=> ( x^2 - 1 )( y^2 - 1 ) > 0
x^2 < 1 => y^2 < 1
=> -1 < y < 1
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