分类：教育/科学-学习帮助
问题：[(xy)/(x
[(x+y)/(x-y)+4xy/(y^2-x^2)]/[（x^2+2xy-3y^2)/(x^2-9y^2)]其中3x^2+xy-2y^2=0
回答：：(x+y)/(x-y)+4xy/(y^2-x^2)     =-(x+y)/(y-x)+4xy/(y^2-x^2)     =-(x+y)(y+x)/(y^2-x^2)+4xy/(y^2-x^2)     =-[(x^2+2xy+y^2-4xy)/(y^2-x^2)]     =-(y-x)^2/(y^2-x^2)     =-(y-x)/(y+x)     =(x-y)/(x+y)分母：（x^2+2xy-3y^2)/(x^2-9y^2)     =(x+3y)(x-y)/(x+3y)(x-3y)     =(x-y)/(x-3y)然后分子/分母得(x-3y)/(x+y) ---------------------------表达式（1）由3x^2+xy-2y^2=0得出[x-（2/3）y](x+y)=0得[x-（2/3）y]=0（即x=（2/3）y）或(x+y)=0根据原表达式可知(x+y)≠0，所以取x=（2/3）y代入：x=（2/3）y ，表达式（1）等于 -7/5