Differential Equations - NCERT

1 Differential EQUATIONS379 He who seeks for methods without having a definite problem in mindseeks for the most part in vain. D. HILBERT IntroductionIn Class XI and in Chapter 5 of the present book, wediscussed how to differentiate a given function f with respectto an independent variable, , how to find f (x) for a givenfunction f at each x in its domain of definition. Further, inthe chapter on Integral Calculus, we discussed how to finda function f whose derivative is the function g, which mayalso be formulated as follows:For a given function g, find a function f such thatdydx =g(x), where y = f(x).

2 (1)An equation of the form (1) is known as a differentialequation. A formal definition will be given Equations arise in a variety of applications, may it be in Physics, Chemistry,Biology, Anthropology, Geology, Economics etc. Hence, an indepth study of differentialequations has assumed prime importance in all modern scientific this chapter, we will study some basic concepts related to Differential equation,general and particular solutions of a Differential equation, formation of differentialequations, some methods to solve a first order - first degree Differential equation andsome applications of Differential Equations in different Basic ConceptsWe are already familiar with the Equations of the type.

3 X2 3x + 3 = (1)sin x + cos x = (2)x + y = (3)Chapter9 Differential EQUATIONSH enri Poincare(1854-1912 )2022-23 MATHEMATICS380 Let us consider the equation:dyxydx+ = (4)We see that Equations (1), (2) and (3) involve independent and/or dependent variable(variables) only but equation (4) involves variables as well as derivative of the dependentvariable y with respect to the independent variable x. Such an equation is called adifferential general, an equation involving derivative (derivatives) of the dependent variablewith respect to independent variable (variables) is called a Differential Differential equation involving derivatives of the dependent variable with respectto only one independent variable is called an ordinary Differential equation, ,3222dydydxdx + =0 is an ordinary Differential (5)

4 Of course, there are Differential Equations involving derivatives with respect tomore than one independent variables, called partial Differential Equations but at thisstage we shall confine ourselves to the study of ordinary Differential Equations onward, we will use the term Differential equation for ordinary differentialequation . shall prefer to use the following notations for derivatives:2323,,dydydyyyydxdxdx === derivatives of higher order, it will be inconvenient to use so many dashesas supersuffix therefore, we use the notation yn for nth order derivative Order of a Differential equationOrder of a Differential equation is defined as the order of the highest order derivative ofthe dependent variable with respect to the independent variable involved in the givendifferential the following Differential Equations .

5 Dydx = (6)2022-23 Differential EQUATIONS38122dyydx+ = (7)332232dydyxdxdx + = (8)The Equations (6), (7) and (8) involve the highest derivative of first, second andthird order respectively. Therefore, the order of these Equations are 1, 2 and 3 Degree of a Differential equationTo study the degree of a Differential equation, the key point is that the differentialequation must be a polynomial equation in derivatives, , y , y , y etc. Consider thefollowing Differential Equations :232322dydydyydxdxdx + + = (9)22sindydyydxdx + = (10)sindydydxdx + = (11)We observe that equation (9) is a polynomial equation in y , y and y , equation (10)is a polynomial equation in y (not a polynomial in y though).

6 Degree of such differentialequations can be defined. But equation (11) is not a polynomial equation in y anddegree of such a Differential equation can not be the degree of a Differential equation, when it is a polynomial equation inderivatives, we mean the highest power (positive integral index) of the highest orderderivative involved in the given Differential 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 ofdifferential equation (11)

7 Is not defined. Note Order and degree (if defined) of a Differential equation are alwayspositive 1 Find the order and degree, if defined, of each of the following differentialequations:(i)cos0dyxdx =(ii) 2220dydydyxyxydxdxdx + = (iii)20yyye ++=Solution(i)The highest order derivative present in the Differential equation is dydx, so itsorder is one. It is a polynomial equation in y and the highest power raised to dydxis one, so its degree is one.(ii)The highest order derivative present in the given Differential equation is 22dydx, soits order is two.

8 It is a polynomial equation in 22dydx and dydx and the highestpower raised to 22dydx is one, so its degree is one.(iii)The highest order derivative present in the Differential equation is y , so itsorder is three. The given Differential equation is not a polynomial equation in itsderivatives and so its degree is not order and degree (if defined) of Differential Equations given in Exercises1 to ()0dyydx += + 5y = += += + ()y + (y )3 + (y )4 + y5 = + 2y + y = 02022-23 Differential + y = + (y )2 + 2y = + 2y + sin y = degree of the Differential equation3222sin10dydydydxdxdx +++= is(A)3(B)2(C)1(D)not order of the Differential equation222230dydyxydxdx += is(A)2(B)1(C)0(D)

9 Not General and Particular Solutions of a Differential EquationIn earlier Classes, we have solved the Equations of the type:x2 + 1 = (1)sin2 x cos x = (2)Solution of Equations (1) and (2) are numbers, real or complex, that will satisfy thegiven equation , when that number is substituted for the unknown x in the givenequation, becomes equal to the consider the Differential equation 220dyydx+=.. (3)In contrast to the first two Equations , the solution of this Differential equation is afunction that will satisfy it , when the function is substituted for the unknown y(dependent variable) in the given Differential equation, becomes equal to curve y = (x) is called the solution curve (integral curve) of the givendifferential equation.

10 Consider the function given byy = (x) =a sin (x + b),.. (4)where a, b R. When this function and its derivative are substituted in equation (3), = So it is a solution of the Differential equation (3).Let a and b be given some particular values say a = 2 and 4b =, then we get afunctiony = 1(x) = 2sin4x + .. (5)When this function and its derivative are substituted in equation (3) = Therefore 1 is also a solution of equation (3).2022-23 MATHEMATICS384 Function consists of two arbitrary constants (parameters) a, b and it is calledgeneral solution of the given Differential equation.