In differential equations, the complete set of solutions is usually formed by the general solution, plus any linear combination of the particular solution(s). If a second-order equation can be written in the form of the above equation, it is called linear, and otherwise nonlinear. An equation involving derivatives of the dependent variable for the independent variable (variables) is known as a differential equation. Order of a differential equation is the order of the highest order derivative occurring in the differential equation. Degree of a differential equation is defined if it is a polynomial equation in its derivatives.