→ Realperson : 感謝 06/23 21:29
※ 引述《Realperson (再大成功也不比家庭)》之銘言:
: 這題苦思許久啊
: 好久沒遇到國中幾何的難題了
: 麻煩挑戰看看了,請再提供您的解法,謝謝
: http://imgur.com/Y33UIUq
設正方形邊長為a, 做輔助線AE
由畢氏定理知
BE = (a^2+4)^(1/2), BF = (a^2-9)^(1/2), AE = [a^2+(a-2)^2]^(1/2)
EF = (AE^2-9)^(1/2) = [a^2+(a-2)^2-9]^(1/2)
且 EF = BE-BF
=> [a^2+(a-2)^2-9]^(1/2) = (a^2+4)^(1/2) - (a^2-9)^(1/2)
兩邊平方
=> a^2+(a-2)^2-9 = 2a^2 -5 -2 [(a^2+4)(a^2-9)]^(1/2)
=> -4a = -2 [(a^2+4)(a^2-9)]^(1/2) 再平方
=> 4a^2 = (a^2+4)(a^2-9) = a^4 -5a^2 -36
=> a^4 -9a^2 -36 = 0
得到 a^2 = 12 或 -3 (負不合)
因此正方型ABCD面積 = a^2 = 12
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