(1)化简:(3a2+2b2)-3(a2-4b2).(2)先化简,再求值:(2a2b+2ab2)-[2(a2b-1)+3ab2+2],其中a=3,b=-2.(3
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(1)化简:(3a2+2b2)-3(a2-4b2). (2)先化简,再求值:(2a2b+2ab2)-[2(a2b-1)+3ab2+2],其中a=3,b=-2. (3)若A=x2-3x-6,B=2x2-4x+6,求:当x=-2时,2A-3B的值. |
答案
(1)原式=3a2+2b2-3a2+12b2 =14b2;
(2)原式=2a2b+2ab2-2a2b+2-3ab2-2 =-ab2, 当a=3,b=-2时,原式=-3×(-2)2=-12;
(3)∵A=x2-3x-6,B=2x2-4x+6, ∴2A-3B=2(x2-3x-6)-3(2x2-4x+6)=2x2-6x-12-6x2+12x-18=-4x2+6x-30, 当x=-2时,2A-3B=-4×(-2)2+6×(-2)-30=-58. |
举一反三
下列各式中成立的是( )A.a+(-2b+c-3d)=a+2b+c-3d | B.a-(-2b+c-3d)=a+2b-c+3d | C.a-2(-2b+c-3d)=a+4b+2c-6d | D.a-2(-2b+c-3d)=a+4b-c+3d |
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