1.若2x³-ax+b可分解為(x+2)(x-1)(2x+c),則a+b+c=?

2015-10-21 1:21 pm

回答 (2)

2015-10-21 1:58 pm
✔ 最佳答案
∵ (x+2)(x-1)(2x+c) = (x^2 +x -2)(2x+c) = 2x^3 + 2x^2 -4x + cx^2 + cx -2c
= 2x^3 +(2+c) x^2 +( c-4) x -2c
i.e. 2x³-ax+b = 2x^3 +(2+c)x^2+( c-4)x -2c

比對系數,
2+ c = 0 => c = -2,
c -4 = -a => -2-4 = -a => a = 6
b = -2c => b =(-2) (-2) = 4

∴ a+b+c= 6+4-2 = 8
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同學:
* 此款數題,多考慮以 「比對系數」作為運算策略!
* 可代入答案: a = 6,b = 4,c = -2 於題目的公式中,作為驗証
2015-10-22 5:01 am
2x^3 -ax+b=(x+2)(x-1)(2x+c)
右方=(x^2-x+2x-2)(2x+c)
右方=(x^2+x-2)(2x+c)
右方=2x^3 +2x^2 -4x+cx^2+cx-2c
右方=2x^3 +(2+c)x^2 +(c-4)x -2c
2+c=0 c-4=-a b=-2c
c=-2 -2-4=-a b=-2(-2)
c=-2 a=-6 b=4
(a,b,c)=(-6,4,-2)
a+b+c
=-6+4-2
=-4


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