Question

a-b=5 ab=--1求(2a+3b-2ab)-（a+4b+ab）-(3ab-2a+2b)的值

Answer 1

(2a+3b-2ab)-（a+4b+ab）-(3ab-2a+2b)
=2a+3b-2ab-a-4b-ab-3ab+2a-2b
=(2-1+2)a+(3-4-2)b+(-2-1-3)ab
=3a-3b-6ab
=3(a-b)-6ab
=3x5-6x(-1)
=21

追问

怎么求ab的值

追答

ab 的值题目中有
单独求a和b,可以解方程组﹛a-b=5 (1)
ab=-1 (2)
(1)得 a=b+5
代入（2）得
（b+5)b=-1
b²+5b+1=0
b=(-5±√21)/2
当b=(-5+√21)/2时，a=(5+√21)/2
当b=(-5-√21)/2时，a=(5-√21)/2

Answer 2

(2a+3b-2ab)-（a+4b+ab）-(3ab-2a+2b)
=2a+3b-2ab-a-4b-ab-3ab+2a-2b
=3a-3b-6ab
=3(a-b)-6ab
=3*5-6*(-1)
=21

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Answer 3

原式=2a+3b-2ab-a-4b-ab-3ab+2a-2b
=3a-3b-6ab

Answer 4

2a+3b-2ab-a-4b-ab-3ab+2a-2b=3a-3b-5ab=3(a-b)-5ab=15+5=20