由x-y除以x+2y等于三分之一,得,
(x-y)/(x+2y)=1/3
3(x-y)=x+2y
3x-3y=x+2y,
3x-x=2y+3y
2x=5y
解得x=5y/2
所以求x-2y除以x+y
=(x-2y)/(x+y)
=(5y/2-2y)/(5y/2+y)
=(y/2)/(7y/2)
=1/7