1.\(A=x^2+y^2+2xy+2x+3y-5\)
\(=\left(x+y\right)^2+2x+3y-5\)
Thay x=1,y=2 vào biểu thức A ta có:
\(\left(x+y\right)^2+2x+3y-5\)
\(=\left(1+2\right)^3+2.1+3.2-5\)
\(=27+2+6-5\)
\(=30\)
2.\(M=x^3+y^3+3xy\)
\(=\left(x+y\right)\left(x^2-xy+y^2\right)+3xy\)
\(=x^2-xy+y^2+3xy\) (vì x+y=1)
\(=\left(x^2+2xy+y^2\right)\)
\(=\left(x+y\right)^2=1\) (vì x+y=1)
3.\(A=x^2+2xy+y^2-4x-4y+1\)
\(=\left(x^2+2y+y^2\right)-\left(4x+4y\right)+1\)
\(=\left(x+y\right)^2-4\left(x+y\right)+1\)
Vì x+y=3, ta có:
\(\left(x+y\right)^2-4\left(x+y\right)+1\)
\(=3^2-4.3+1\)
\(=9-12+1\)
\(=-2\)
