(x+y/x-y + 4xy/y2-x2)(x2-9y2)/（x2+2xy-3y2）
=[(x+y/x-y -4xy/(x+y)(x-y)](x+3y)(x-3y)/(x+3y)(x-y).(平方差公式、十字相乘法）
=[（x-y）的平方/(x+y)(x-y)](x+3y)(x-3y)/(x+3y)(x-y).(通分,完全平方公式）
=（x-3y)/(x+y).（约分,化简）