設正實數a,b滿足(1/a)-(1/b)-(1/(a+b))=0,則(a/b)^3-(b/a)^3=? 已知答案為-4,求詳解,謝謝!?

2017-10-04 3:17 pm

回答 (2)

2017-10-04 6:22 pm
✔ 最佳答案
Sol
p=a/b
1/a-1/b-1/(a+b)=0
b(a+b)-a(a+b)-ab=0
ab+b^2-a^2-ab-ab=0
-a^2-ab+b^2=0
a^2+ab-b^2=0
(a/b)^2+(a/b)-1=0
p^2+p-1=0
p^2=1-p
p^3=p-p^2=p-(1-p)=2p-1
p^6=4p^2-4p+1=4(1-p)-4p+1=5-8p
(a/b)^3-(b/a)^3
=p^3-1/p^3
=(p^6-1)/p^3
=(4-8p)/(2p-1)
=-4
/
2017-10-04 6:07 pm
(1/a)-(1/b)-(1/(a+b))=0
=> (1/a) - (1/b) = 1/(a+b)
=> (b-a) / (ab) = 1/(a+b)
=> (b-a)(a+b) / ab = 1
=> (b²-a²) / (ab) = 1 . . . . . . . . . . . . . ①

[(a/b) - (b/a)]³ = (a/b)³ - 3(a/b)²(b/a) + 3(a/b)(b/a)² - (b/a)³
(a/b)³ - (b/a)³ = [(a/b) - (b/a)]³ + 3(a/b) - 3(b/a)
      = [(a/b) - (b/a)]³ + 3[(a/b) - (b/a)]
      = [(a²-b²) / (ab)]³ + 3[(a²-b²) / (ab)]
      = (-1)³ + 3(-1) . . . . . . . . . . . . ( 從 ① )
      = - 4

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 完 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

小提示 :
=====
當有 :i) 已知條件 : (1/a)-(1/b)-(1/(a+b))=0,及 ii) 要求的數式 : (a/b)^3-(b/a)^3 =? 時;
可考慮 從兩邊 i) & ii) 都同時 入手,
就比較容易 找出線索 / 解答方案!


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