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第229行： − 微分方程是用它们的阶来描述的，由导数最高的项来确定。只含有一阶导数的方程是一阶微分方程，含有二阶导数的方程是二阶微分方程，等等。描述自然现象的微分方程几乎总是只有一阶和二阶导数，但也有一些例外，例如薄膜方程，它是一个四阶偏微分方程。+
→{{anchor|Second order}} Equation order
==={{anchor|Second order}} Equation order===
==={{anchor|Second order}} Equation order===
方程的阶
Differential equations are described by their order, determined by the term with the [[Derivative#Higher derivatives|highest derivatives]]. An equation containing only first derivatives is a ''first-order differential equation'', an equation containing the [[second derivative]] is a ''second-order differential equation'', and so on.<ref>[[Eric W Weisstein|Weisstein, Eric W]]. "Ordinary Differential Equation Order." From [[MathWorld]]--A Wolfram Web Resource. http://mathworld.wolfram.com/OrdinaryDifferentialEquationOrder.html</ref><ref>[http://www.kshitij-iitjee.com/Maths/Differential-Equations/order-and-degree-of-a-differential-equation.aspx Order and degree of a differential equation] {{Webarchive|url=https://web.archive.org/web/20160401070512/http://www.kshitij-iitjee.com/Maths/Differential-Equations/order-and-degree-of-a-differential-equation.aspx |date=2016-04-01 }}, accessed Dec 2015.</ref> Differential equations that describe natural phenomena almost always have only first and second order derivatives in them, but there are some exceptions, such as the [[Thin-film equation|thin film equation]], which is a fourth order partial differential equation.
Differential equations are described by their order, determined by the term with the [[Derivative#Higher derivatives|highest derivatives]]. An equation containing only first derivatives is a ''first-order differential equation'', an equation containing the [[second derivative]] is a ''second-order differential equation'', and so on.<ref>[[Eric W Weisstein|Weisstein, Eric W]]. "Ordinary Differential Equation Order." From [[MathWorld]]--A Wolfram Web Resource. http://mathworld.wolfram.com/OrdinaryDifferentialEquationOrder.html</ref><ref>[http://www.kshitij-iitjee.com/Maths/Differential-Equations/order-and-degree-of-a-differential-equation.aspx Order and degree of a differential equation] {{Webarchive|url=https://web.archive.org/web/20160401070512/http://www.kshitij-iitjee.com/Maths/Differential-Equations/order-and-degree-of-a-differential-equation.aspx |date=2016-04-01 }}, accessed Dec 2015.</ref> Differential equations that describe natural phenomena almost always have only first and second order derivatives in them, but there are some exceptions, such as the [[Thin-film equation|thin film equation]], which is a fourth order partial differential equation.
Differential equations are described by their order, determined by the term with the highest derivatives. An equation containing only first derivatives is a first-order differential equation, an equation containing the second derivative is a second-order differential equation, and so on. Differential equations that describe natural phenomena almost always have only first and second order derivatives in them, but there are some exceptions, such as the thin film equation, which is a fourth order partial differential equation.
Differential equations are described by their order, determined by the term with the highest derivatives. An equation containing only first derivatives is a first-order differential equation, an equation containing the second derivative is a second-order differential equation, and so on. Differential equations that describe natural phenomena almost always have only first and second order derivatives in them, but there are some exceptions, such as the thin film equation, which is a fourth order partial differential equation.
微分方程是用它们的阶来描述的，并且是由导数最高的项来确定。只含有一阶导数的方程是一阶微分方程，含有二阶导数的方程是二阶微分方程，等等。描述自然现象的微分方程几乎总是只有一阶和二阶导数，但也有一些例外，例如薄膜方程，它是一个四阶偏微分方程。
===Examples===
===Examples===