Question

若x-2y=15,xy=-25,求x^2+4y^2-1的值

Answer 1

x-2y=15,xy=-25,

(x-2y)^2

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

=x^2+4y^2+100=15^2=225

x^2+4y^2=225-100=125

x^2+4y^2-1=124