Re: [中學] 解方程式
※ 引述《mariowu (老馬也會識途)》之銘言:
: 大家好 小弟我最近遇到一個題目
: x^2 - 4xy + 5y^2 + 2x - 8y + 5 = 0
: 問x*y=?
: 目前有想過要雙十字交乘但配不出來
: 況且x、y並沒有給其他條件,一個式子算不出兩個未知數
: 想問版上的大大這題要怎麼辦?
x^2 - 4xy + 4y^2 + y^2 + 2x - 8y + 5 = 0
(x-2y-1)^2 + y^2 - 4y + 4 = 0
(x-2y+1)^2 + (y-2)^2 = 0 --> y = 2, x = 3
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