[微分] 101中正 函數恰有一解
題目:
For what value of C does the equation
ln x = Cx^3 have exactly one solution?
解答:
----------------
先畫出 ln x 與 Cx^3 的圖形
然後發現
當 C < 0 一定跟 ln x 相交
討論 C > 0
令 F(x) = Cx^3-lnx
F'(x) = 3Cx^2 - 1/x = 0
>> C = 1 / 3x^3
代回 f(x) 解得 C = 1 / 3e
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想問版上大大 一看到題目
要怎麼直觀的想到
C > 0
滿足 F'(x) = 0 的條件
將會有唯一解,
另外為什麼C < 0 ,卻又要用圖形判斷?
而不是 從 F'(x) = 0 看出
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