xdy - 2ydx = 0,
xdy = 2ydx,
显然y = 0是1个解.
当 x 不等于0,且y不等于0时,
dy/y = 2dx/x,
ln|y| = lnx^2 + C,C为任意常数,
|y| = exp{lnx^2 + C} = Dexp{lnx^2} = Dx^2,D 为任意正数.
所以,
y = cx^2,c 为任意常数.