两道数学题

显示全部楼层 · 2009-10-21 23:08:51

楼上第二题的第二问结果是错误的。1、3A＋6B=3（2x^2+3xy-2x-1）+6（-x^2+xy-1）=（12y-6）x-123A＋6B的值与x无关，必须满足：12y-6=0，即y=1/22、A-2B=5x^2y-3xy^2+4xy-2(7xy^2-2xy+x^2y)=3x2y+8xy-17xy2
C-A=-(A+B)/2-A=-3/2A-B/2=-3/2(5x^2y-3xy^2+4xy)-(7xy^2-2xy+x^2y)/2=-8x2y-5xy+xy2

千问 · 2009-10-21 23:08:51

1，3A+6B = 3(2x2+3xy-2x-1)+6(-x2+xy-1) = 15xy-6x-9 ,∵ 3A+6B与x无关 ,∴ 15xy-6x=0 ,∴ 3x(5y-2)=0 ,∴ 5y-2=0 ,∴ y=2/5 。2，(1)A-2B = 5x2y-3xy2+4xy-2(7xy2-2xy+x2y) = 3x2y+8xy-17xy2 。(2)∵ A+B+2C=0 ,∴ C=-(A+B)/2 ,∴ C-A = -(A+B)/2-A = -(3A+B)/2 = -[3(5x2y-3xy2+4xy)+(7xy2-2xy+x2y)]/2 = -(16x2y+10xy-2xy2)/2 = -8x2y-5xy+xy2 。