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電子課本網 第173頁

第173頁

信息發(fā)布者:
?$解:原式=2m^2-4m+1-2m^2-4m+1$?
?$=-8m+2,$?
?$將m=-2代入得原式=18.$?
?$解:原式=2a^2-4ab+\frac{1}{2}a^2-2a^2+b^2=-\frac72ab$?
?$將a=-2,b= 3代入得原式=21.$?
?$解:原式=\frac13a-\frac12a+4b+6c-6c+6b$?
?$=-\frac 16a+10b$?
?$將a=-18,b=\frac{1}{5}代入得原式=5.$?
?$解:原式=4xy-(x^2+5xy-y^2-x^2-3xy+2y^2)$?
?$=4xy-(2xy+y^2)$?
?$=4xy-2xy-y^2$?
?$=2xy-y^2,$?
?$因為(x-1)^2+|y+\frac{1}{2}|=0,$?
?$所以x=1,y=-\frac{1}{2},$?
?$代入得原式=-\frac{5}{4}$?

?$解:5x-8-[2(x-y)-4]$?
?$=5x-8y-(2x-2y-4)$?
?$=3x-6y+4$?
?$=3(x-2y)+4$?
?$=3+4=7,$?
?$故5x-8y\gt 2(x-y)-4.$?
?$解:(1)2A-B=2(x^2+xy+3y)-(x^2-xy)$?
?$=2x^2+2xy+6y-x^2+xy$?
?$=x^2+3xy+ 6y,$?
?$把x=-2,y=5代入得原式=4$?
?$(2)由(1)得2A-B=x^2+3xy+6y=x^2+(3x+6)y,$?
?$因為2A-B的值與y的值無關,$?
?$所以3x+6=0,$?
?$解得x=-2.$?
?$解:(1)原式=x^3+bx^2-1-2ax^3+x^2-x$?
?$=(1-2a)x^3+(b+1)x^2-x-1,$?
?$由題意得1-2a=0且b+1=0,$?
?$解得a=\frac{1}{2},b=-1.$?
?$(2)原式=2a^2-2ab+2-2a^2+6ab-12$?
?$=4ab-10,$?
?$當a=\frac{1}{2},b=-1時,$?
?$原式=4×\frac{1}{2}×(-1)-10=-12.$?