\(A-\left(3xy-4y^2\right)=x^2-7xy+8y^2\)

\(A=x^2-7xy+8y^2+\left(3xy-4y^2\right)\)

\(A=x^2-7xy+8y^2+3xy-4y^2\)

\(A=x^2-\left(7xy-3xy\right)+\left(8y^2-4y^2\right)\)

\(A=x^2-4xy+4y^2\)

\(A=\left(x-2y\right)^2\)

\(A-\left(3xy-4y^2\right)=x^2-7xy+8y^2\)

=>A=\(x^2-4xy+4y^2\)

=>A=\(\left(x-2y\right)^2\)

Vậy A=\(\left(x-2y\right)^2\)

a/

 \(M+5x^2-2xy-6x^2-9xy+y^2=0\)

\(M-x^2-11xy+y^2=0\)

\(M-x^2+y^2-11xy=0\)

\(M=x^2-y^2+11xy\)

Vậy:..

Câu b tương tự

M + (5x² - 2xy) = 6x² + 9xy - y^{2   ta có} M = ( 6x² + 9xy - y^{2) -(}5x² - 2xy)= x2 +11xy -y2

\(M=6x^2+9xy-y^2-5x^2+2xy\)

\(M=x^2+11xy-y^2\)

\(N=3xy-4y^2-x^2+7xy-8y^2\)

\(N=-x^2+10xy-12y^2\)

a.    \(M+\left(5x^2-2xy\right)=6x^2+9xy-y^2\)

\(\Rightarrow M=6x^2+9xy-y^2-5x^2+2xy\)

b.     \(\left(3xy-4y^2\right)-N=x^2-7xy+8y^2\)

\(\Rightarrow N=3xy-4y^2-x^2+7xy-8y^2\)

a, \(A=-x^2+4xy^2-2xz+3y^2\)

b, \(B=6x^2+9xy-y^2-5x^2+2xy=x^2+11xy-y^2\)

c, \(A=3xy-4y^2-x^2+7xy-8y^2=-x^2+10xy-12y^2\)

a, \(M-\left(3xy-4y^2-2xy\right)=\left(x^2-7xy+8y^2\right)\)

\(\Rightarrow M=\left(x^2-7xy+8y^2\right)+\left(3xy-4y^2-2xy\right)\)

\(\Rightarrow M=x^2-7xy+8y^2+3xy-4y^2-2xy\)

\(\Rightarrow M=x^2+\left[3xy-7xy-2xy\right]+\left[8y^2-4y^2\right]\)

\(\Rightarrow M=x^2-6xy+4y^2\)

b, \(N+\left(x^3-xyz+3x^2y\right)=2x^3+3xy-xy^2\)

\(\Rightarrow N=\left(2x^3+3xy-xy^2\right)-\left(x^3-xyz+3x^2y\right)\)

\(\Rightarrow N=2x^3+3xy-xy^2-x^3+xyz-3x^2y\)

\(\Rightarrow N=\left[2x^3-x^3\right]+3xy-xy^2+xyz-3x^2y\)

\(\Rightarrow N=x^3+3xy-xy^2+xyz-3x^2y\)

Tích mình nha!!!

a: \(M=6x^2+9xy-y^2-5x^2+2xy=x^2+11xy-y^2\)

b: \(N=3xy-4y^2-x^2+7xy-8y^2=-x^2+10xy-12y^2\)

a)  A = 6x² + 9xy - y² + 5x² + 2xy 

          = (6x² + 5x²) +(9xy + 2xy) -y²

          = 11x² + 11xy - y²

Học tốt! 

để mk lm sai

A+(5x^2-2xy)=6x^2+9xy-y^2

 =>A= (6x^2+9xy-y^2)-(5x^2-2xy)

       =6x^2+9xy-y^2+5x^2+2xy

        = (6x^2+5x^2)+ (9xy+2xy)-y^2

       =11x^2+11xy-y^2

b)(3xy-4y^2)-B = x^2-7xy+8y^2

 =>B=(3xy-4y^2)-( x^2-7xy+8y^2)

      = 3xy-4y^2+x^2+7xy-8y^2

      = (3xy+7xy)-(4y^2+8y^2)+x^2