Question

已知x(x-1)-(x²-y)=3,求代数式(x²+y²)／2-xy的值

Answer 1

因x(x-1)-(x²-y)=3,所以x²-x-x²+y=3,y-x=3

又(x²+y²)／2-xy=（x²+2xy+y²）/2=(x-y)^2/2=3^2/2=9/2