Question

已知x+y=a，x^2+y^2=b，求4x^2y^2的值

Answer 1

x+y=a 两边平方得X^2+2xy+y^2=a^2, 而x^2+y^2=b得2xy=a^2-b，则4x^2y^2=(a^2-b)^2

Answer 2

x+y=a
(x+y)^2=a^2
x^2+2xy+y^2=a^2
b+2xy=a^2
2xy=a^2-b
4x^2y^2=(2xy)^2=(a^2-b)^2=a^4-2a^2b+b^2