(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怎么求ab的值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=-1b²+5b+1=0b=(-5±√21)/2当b=(-5+√21)/2时，a=(5+√21)/2当b=(-5-√21)/2时，a=(5-√21)/2