Question

已知x+y=5,则x(x-y)+5y(x+y)+(x^3-3y^2)的值是

Answer 1

已知x+y=5,则x(x-y)+5y(x+y)+(x^2-3y^2)的值是

x(x-y)+5y(x+y)+(x^2-3y^2)

= x ^2 - xy + 5xy + 5y^2 + x^2 - 3y^2

= 2x ^2 + 4xy + 2y^2

= 2 ( x+y ) ^2

= 2x 25

= 50

选D