Mathematics Introduction Basic Concepts Order of a differential equation Degree of a differential equation For CBSE-NCERT
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color {red} **  Introduction
color {red} **  Basic Concepts
color {red} **  Order of a differential equation
color {red} **  Degree of a differential equation

Introduction

=> In general, an equation involving derivative (derivatives) of the dependent variable with respect to independent variable (variables) is called a differential equation.

=> A differential equation involving derivatives of the dependent variable with respect to only one independent variable is called an ordinary differential equation, e.g.,

color{orange} {2 (d^y)/(dx^2) + ((dy)/(dx) )^3 = 0} is an ordinary differential equation.

Note : 1. . We shall prefer to use the following notations for derivatives:

color{blue}{((dy)/(dx)) = y' ;\ \ ((d^2y)/(dx^2)) = y'' ;\ \((d^3y)/(dx^3)) = y''' }

2. For derivatives of higher order, it will be inconvenient to use so many dashes as supersuffix therefore, we use the notation y_n

for n^(th) order derivative color{blue}{((d^n y)/(dx^n)).}

Order of a differential equation

=> Order of a differential equation is defined as the order of the highest order derivative of the dependent variable with respect to the independent variable involved in the given differential equation.

Consider the following differential equations:

color{orange} {(dy)/(dx) = e^x} .......(6)

color{orange} {(d^2y)/(dx^2) + y = 0}.........(7)

color{orange} {((d^3y)/(dx^3) ) + x^2 ((d^2y)/(dx^2))^3 = 0} .........(8)

=> The equations (6), (7) and (8) involve the highest derivative of first, second and third order respectively.

Degree of a differential equation

=> To study the degree of a differential equation, the key point is that the differential equation must be a polynomial equation in derivatives, i.e., y′, y″, y″′ etc.

Consider the following differential equations:

color{orange} {(d^3y)/(dx^3) + 2 ((d^2y)/(dx^2))^2 - (dy)/(dx) + y = 0} .......(9)

color{orange} {((dy)/(dx))^2 + ((dy)/(dx)) - sin^2 y = 0}.......(10)

color{orange} {(dy)/(dx) + sin ((dy)/(dx)) = 0} .........(11)

=> By the degree of a differential equation, when it is a polynomial equation in derivatives, we mean the highest power (positive integral index) of the highest order derivative involved in the given differential equation.

=> In view of the above definition, one may observe that differential equations (6), (7), (8) and (9) each are of degree one, equation (10) is of degree two while the degree of differential equation (11) is not defined.

Note : Order and degree (if defined) of a differential equation are always positive integers.
Q 3156080874

Find the order and degree, if defined, of each of the following differential equations:

(i) (dy)/(dx) - cos x = 0

(ii) xy (d^2y)/(dx^2) + x ((dy)/(dx))^2 - y (dy)/(dx) =0

(iii) y''' + y^2 + e^(y') = 0
Class 12 Chapter 9 Example 1
Solution:

(i) The highest order derivative present in the differential equation is (dy)/(dx), so its order is one. It is a polynomial equation in y′ and the highest power raised to (dy)/(dx) is one, so its degree is one.

(ii) The highest order derivative present in the given differential equation is (d^2y)/(dx^2), so its order is two. It is polynomial equation in (d^2y)/(dx^2) and (dy)/(dx) and the highest power raised to (d^2y)/(dx^2)  is one, so its degree is one.

(iii) The highest order derivative present in the differential equation is y′′′ , so its order is three. The given differential equation is not a polynomial equation in its derivatives and so its degree is not defined.