X^2-xy+y^2=(x-y)^2=(1/2根号7+1/2根号5-1/2根号7+1/2根号5）^2=(根号5）^2=5
x^2-xy+y^2=(x-y)^2+xyx-y=根号5 xy=1/2所以原式=(x-y)^2+xy=（根号5）^2+1/2=11/2
x2-xy+y2=x2-xy-xy+y2+xy=(x-y)2+xy.因为x-y=根号5.xy=1/2.所以x2-xy+y2=5+1/2=5.5
