(x+y)/(x)=3
1+(y/x)=3
得:y/x=2
则:
(x²+y²)/(x²-xy+y²) 【分子分母同除以x²】
=[1+(y/x)²]/[1-(y/x)+(y/x)²]
=[1+2²]/[1-2+2²]
=5/3