Determine order and degree (when defined) of differential equations.
d4ydx4+sin(y′′′)=0
d4ydx4+sin(y′′′)=0⇒y′′′′+sin(y′′′)=0
The highest order derivative which occurs in the given differential equations y'''', therefore its order is 4.
As the given differential equation is not a polynomial equation in dy/dx (i.e., y') therefore its degree is not defined.