Re: [中學] 兩題指對數
重新整理過後變成 (x + log_2(a) - 8)^2 = 16 [ 4 - log_2(a) ]
因為有兩相異實根，所以 log_2(a) < 4 ， a < 16
但別忘了 a 一定要大於 0 ，故 0 < a < 16
: 順便問一下下面這題的解為什麼不是25大於等於k大於0
: http://i.imgur.com/PX7YESY.jpg
: 感謝大家
令 t = 10^x，原式 = t^2 - 10t + k = 0
(t-5)^2 = 25 - k ， 故 k ≦ 25
因為兩根均為正數， t > 1 ， -sqrt(25-x) > 1-5 = -4
=> 25-x < 16 => x > 9
故 9 < x ≦ 25
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※ 編輯: Eliphalet (114.46.213.135), 05/29/2015 12:16:48
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