Bài 26 :
\(a,A+B+C=4x^2-5xy+3y^2+3x^2+2xy+y^2-x^2+3xy+2y^2\)
\(=6x^2+6y^2\)
\(b,-A+B-C=-4x^2+5xy-3y^2+3x^2+2xy+y^2+x^2-3xy-2y^2\)
\(=4xy-4y^2\)
\(c,-A-B+C=-4x^2+5xy-3y^2-3x^2-2xy-y^2-x^2+3xy+2y^2\)
\(=-8x^2+6xy-2y^2\)
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Bài 27 :
\(a,M=6x^2+9xy-y^2-\left(5x^2-2xy\right)=6x^2+9xy-y^2-5x^2+2xy\)
\(=x^2+11xy-y^2\)
\(b,M=x^2-7xy+8y^2+\left(3xy-4y^2\right)=x^2-7xy+8y^2+3xy-4y^2\)
\(=x^2-4xy+4y^2=\left(x-2y\right)^2\)
\(c,M=25x^2y-13xy^2+y^3-11x^2y+2y^2\)
\(=14x^2y-13xy^2+y^3+2y^2\)
\(d,M=-\left(12x^4-15x^2y+2xy^2+7\right)=-12x^4+15x^2y-2xy^2-7\)
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