比例 急!!!!!

2008-05-13 1:47 am
已知:
a+b-c a-b+c -a+b+c
-----=-----=------,
  c     b      a
求:
(a+b)(b+c)(c+a)
---------------的值。
      abc


thx

回答 (2)

2008-05-13 2:40 am
✔ 最佳答案
幾得意
首先第 1 舊乘 ab/ab
第 2 舊乘 ac/ac
第 3 舊乘 bc/bc

得出
( a^2 * b + a * b^2 – abc ) / abc = ( a^2 * c + a * c^2 – abc ) / abc = ( b^2 * c + b * c^2 – abc ) / abc

分2個 case

第1個 case 係 a, b, c 3 個數中有 2 個或 3 個唔一樣
就當 b =/= c

咁用左邊2個
( a^2 * b + a * b^2 – abc ) / abc = ( a^2 * c + a * c^2 – abc ) / abc

a^2 * b + a * b^2 – abc = a^2 * c + a * c^2 – abc
[a, b, c 3 個都唔係 0, 約得]

a^2 * b + a * b^2 = a^2 * c + a * c^2

a^2 * b – a^2 * c = a * c^2 – a * b^2
[ 改次序 ]

a^2 * (b – c) = a (c^2 – b^2)

a * (b – c) = (c – b) (c + b)
[約左 a 同埋因式分解]

a = - (c + b)
[ 假設左 b=/= c, 所以 (b - c) 約得 ]

b + c = -a
即 a + b + c = 0

咁就好辦
(a + b) (b + c) (c + a) / (abc)
= (-c) (-a) (-b) / (abc)
= -1

又如果 case 2, 3個數都一樣
(a + b) (b + c) (c + a) / (abc)
= 8a^3 / a^3
= 8
2008-05-13 2:10 am
a+b-c a-b+c -a+b+c
-----=-----=------ = k
  c     b      a
a+b   c   a+c   b  b+c   a
--- - - = --- - - =--- - -= k
 c    c    b    b   a    a

a+b       a+c      b+c   
--- - 1 = --- - 1 =--- - 1= k
 c         b        a    
a+b   a+c   b+c
--- = --- = --- = k+1
 c     b     a 
(a+b)(b+c)(c+a)
--------------- 
      abc       
 a+b   b+c   a+c
=--- x --- x ---
  c     a     b
=(k+1)^3
= k^3 + 3k^2 + 3k + 1//


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