1/x-1/y=1/3(2x+3xy-2y)/(x-3xy-y) 分子分母同除以xy：= {2/y+3-2/x} / {1/y-3-1/x}= {-2(1/x-1/y)+3} /{ -(1/x-1/y)-3}= {-2*1/3+3} / {-1/3-3}= {-2+9} / {-1-9}= -7/10