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

\(=\left(x+y\right)^3+2\left(x^2+2xy+y^2\right)\)

\(=\left(x+y\right)^3+2\left(x+y\right)^2\)

\(=7^3+2\cdot7^2=441\)

Trừ 2 vế ta đc : 2y = 8 => y = 4

Thay y = 4 vào x + y = 5 ta đc:

x + 4 = 5 => x = 1

Vậy x = 1 ; y = 4

\(C=4x^2-12x-9\)

\(=4x^2-12x+9-18\)

\(=\left(2x-3\right)^2-18\ge-18\)

Khi \(x=\dfrac{3}{2}\)