Re: [微積] 有兩條證明在區間在在有解的問題
: 是兩條題目, 題7我大概知道方向, 但不知道對不對.
: 題8我就完全沒有方向, 要向各位請教了.
: 謝謝各位!
8.Show that f(x)=x^4+8x^3+24x^2-12 = 0 has exactly two roots in (-∞,+∞)
pf: (i) f(1) = 1+8+24-12>0
f(0) = -12<0 => there are at least two roots in
f(-1)= 1-8+24-12>0 (0,1) & (-1,0) respectively
(ii) Suppose that there are three roots for equation f(x)=0
Assume that they are x,y,z such that f(x)=f(y)=f(z)=0
By Rolle's Thm, there exists c_1 & c_2 so that
f'(c_1)=f'(c_2)=0
However, f'(x) = 4x^3+24x^2+48x =0 has only one roots
a contradiction
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◆ From: 111.249.15.155
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11/07 22:02, , 1F
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※ 編輯: cxcxvv 來自: 111.249.15.155 (11/07 22:20)
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11/07 22:10, , 6F
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*[1;31m→ playmypig :一實根. 這樣可以嗎? 11/07 22:11
這樣應該沒問題了XD
※ 編輯: cxcxvv 來自: 111.249.15.155 (11/07 22:22)
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11/07 22:30, , 7F
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