Question

已知（x+y)的平方=20,（x+y)的平方=40,求X的平方+Y的平方和XY的值

Answer 1

(x+y)^2-(x-y)^2=40-20

x^2+2xy+y^2-(x^2-2xy+y^2)=20

x^2+2xy+y^2-x^2+2xy-y^2=20

2xy=20

xy=10

(x+y)^2=40

x^2+2xy+y^2=40

x^2+y^2=40-2xy=40-2×10=20