※ 引述《wellilin (QQ12)》之銘言：
: (y^4+2y)dx+(xy^3+2y^4-4x)dy=0
: ans:xy+y^2+2x/y^2=c
y^4dx + 2ydx +xy^3dy +2y^4dy -4xdy = 0
y^3(ydx+xdy) + 2(ydx-2xdy) + 2y^4dy = 0
y^3 dxy + 2 y^3 d(x/y^2) + 2y^4dy = 0
同除y^3
dxy + 2 d(x/y^2) + 2ydy = 0
xy + 2(x/y^2) + y^2 = c
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