#)Giải :
a)\(A=x^2+2xy+y^2-4x-4y+1=\left(x^2+2xy+y^2\right)-4\left(x+y\right)+1=\left(x+y\right)^2-4\left(x+y\right)+1\)
Thay x + y = 3 vào biểu thức, ta được : \(A=3^2-4.3+1=-2\)
c) \(C=3\left(x^2+y^2\right)-\left(x^3+y^3\right)+1\)
\(=3x^2+3y^2-\left(x+y\right)\left(x^2-xy+y^2\right)+1\)
\(=3x^2+3y^2-2\left(x^2-xy+y^2\right)+1\)
\(=3x^2+3y^2-2x^2+2xy-2y^2+1\)
\(=x^2+y^2+2xy+1\)
\(=\left(x+y\right)^2+1=2^2+1=5\)
b) \(B=x^2-2xy+y^2-5x+5y+6\)
\(=\left(x-y\right)^2-5\left(x-y\right)+6\)
\(=7^2-5.7+6\)
\(=49-29=20\)