微分方程dx/2(x+y^4)=dy/y
两边积分得 1/2㏑(x+y^4)=lny+lnc
化简的 x=cy^2-y^4
这样做,什么地方错了吗?
人气:212 ℃ 时间:2020-01-30 03:22:06
解答
解法一:(全微分法)
∵dx/2(x+y^4)=dy/y ==>ydx=2(x+y^4)dy
==>ydx-2xdy=2y^4dy
==>(ydx-2xdy)y³=2ydy
==>d(x/y²)=d(y²)
==>x/y²=y²+C(C是积分常数)
==>x=(y²+C)y²
∴原微分方程的通解是x=(y²+C)y²(C是积分常数);
解法二:(常数变易法)
∵齐次方程ydx=2xdy ==>dx/x=2dy/y
==>ln│x│=2ln│y│+ln│C│(C是积分常数)
==>x=Cy²
∴此齐次方程的通解是x=Cy²(C是积分常数)
于是,设原微分方程的解为x=C(y)y²(C(y)表示关于y的函数)
∵代入原方程,化简整理得C'(y)y³=2y^4
==>C'(y)=2y
==>C(y)=y²+C(C是积分常数)
==>x=C(y)y²=(y²+C)y²
∴原微分方程的通解是x=(y²+C)y²(C是积分常数).
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