Differential Equations Question Answers: NCERT Class 12 Mathematics

Exercise 1
Q:
A:

\begin{align} \frac{d^4y}{dx^4}\;+\;\sin(y^m)\;=0 \end{align}

\begin{align} \Rightarrow y^{m\;'}+\;\sin(y^m)\;=0 \end{align}

The highest order derivative present in the differential equation is ym '. Therefore, its order is four.

The given differential equation is not a polynomial equation in its derivatives. Hence, its degree is not defined.


Q:
A:

yn + 2y' + siny = 0

The highest order derivative present in the differential equation is yn. Therefore, its order is two.

This is a polynomial equation in yand y' and the highest power raised to yis one. Hence, its degree is one.


Q:
A:

\begin{align}\left(\frac{d^2y}{dx^2}\right)^3\;+ \left(\frac{dy}{dx}\right)^2+\;sin\left(\frac{dy}{dx}\right)\;+ 1=\;0\end{align}

The given differential equation is not a polynomial equation in its derivatives. Therefore, its degree is not defined.

Hence, the correct answer is D.


Q:
A:

\begin{align}2x^2\frac{d^2y}{dx^2}\;- \;3\frac{dy}{dx}\;+ y=\;0\end{align}

The highest order derivative present in the given differential equation is

\begin{align}\frac{d^2y}{dx^2}\end{align}

Therefore, its order is two.

Hence, the correct answer is A.


Q:
A:

The given differential equation is:

y' + 5y = 0

The highest order derivative present in the differential equation isy'. Therefore, its order is one.

It is a polynomial equation in y'. The highest power raised to y' is 1. Hence, its degree is one.


Q:
A:

\begin{align}\left(\frac{ds}{dt}\right)^4\;+\;3s\frac{d^2s}{dt^2}\;=\;0\end{align}

The highest order derivative present in the given differential equation is\begin{align}\frac{d^2s}{dt^2}.\end{align}

 Therefore, its order is two. It is a polynomial equation in

\begin{align}\frac{d^2s}{dt^2} and \frac{ds}{dt}.\end{align}

The power raised to is 1.  \begin{align} \frac{d^2s}{dt^2} \end{align}


Q:
A:

\begin{align}\left(\frac{d^2y}{dx^2}\right)^2\;+\;cos\left(\frac{dy}{dx}\right)\;=\;0\end{align}

The highest order derivative present in the given differential equation is \begin{align}\frac{d^2y}{dx^2}.\end{align}

Therefore, its order is 2. The given differential equation is not a polynomial equation in its derivatives.

Hence, its degree is not defined.


Q:
A:

\begin{align}\frac{d^2y}{dx^2}=\cos3x  + sin3x\end{align}

\begin{align}\Rightarrow\frac{d^2y}{dx^2} - \cos3x  - sin3x = 0\end{align}

The highest order derivative present in the differential equation is\begin{align}\frac{d^2y}{dx^2}.\end{align}

Therefore, its order is two.It is a polynomial equation in \begin{align}\frac{d^2y}{dx^2}\end{align} 

and the power raised to is 1. 

\begin{align}\frac{d^2y}{dx^2}\end{align}

Hence, its degree is one.


Q:
A:

(ym)2 + (yn)3 + (y')4 + y5 =0

The highest order derivative present in the differential equation isym. Therefore, its order is three.

The given differential equation is a polynomial equation in ym ,  yn , y'.

The highest power raised to yis 2. Hence, its degree is 2.


Q:
A:

The highest order derivative present in the differential equation is ym. Therefore, its order is three.

It is a polynomial equation in ym , yn  and y' . The highest power raised to ym is 1. Hence, its degree is 1.


Q:
A:

y' + y =ex

y' + y  - e=0

The highest order derivative present in the differential equation is y'. Therefore, its order is one.

The given differential equation is a polynomial equation in yand the highest power raised to yis one. Hence, its degree is one.


Q:
A:

yn + (y')2 + 2y =0

The highest order derivative present in the differential equation is yn. Therefore, its order is two.

The given differential equation is a polynomial equation in yand y' and the highest power raised to yis one.

Hence, its degree is one.


Exercise 2
Q:
A:

y = ex +1

Differentiating both sides of this equation with respect to x, we get:

\begin{align}\frac{dy}{dx}=\frac{d}{dx}(e^x + 1)\end{align}

=> y' = ex                          ...(1)

Now, differentiating equation (1) with respect to x, we get:

\begin{align}\frac{d}{dx}(y^{'})=\frac{d}{dx}(e^x)\end{align}

=> y'' = ex

Substituting the values of y' and y'' in the given differential equation, we get the L.H.S. as:

y'' - y' = ex - ex = 0 = R.H.S.

Thus, the given function is the solution of the corresponding differential equation.


Q:
A:

y = x2 + 2x + C

Differentiating both sides of this equation with respect to x, we get:

\begin{align}y^{'}=\frac{d}{dx}(x^2 + 2x + C)\end{align}

=> y' = 2x + 2

Substituting the value of y' in the given differential equation, we get:

L.H.S. = y' - 2x - 2 = 2x + 2 - 2x - 2 = 0 = R.H.S.

Hence, the given function is the solution of the corresponding differential equation.


Q:
A:

y = cosx + C

Differentiating both sides of this equation with respect to x, we get:

\begin{align}y^{'}=\frac{d}{dx}(cosx + C)\end{align}

=> y' = - sinx

Substituting the value of y'  in the given differential equation, we get:

L.H.S. = y' + sinx = - sinx + sinx = 0 = R.H.S.

Hence, the given function is the solution of the corresponding differential equation.


Q:
A:

\begin{align} y= \sqrt{1+x^2}\end{align}

Differentiating both sides of the equation with respect to x, we get:

\begin{align}  y^{'}=\frac{d}{dx}\left(\sqrt{1+x^2} \right)\end{align}

\begin{align}  y^{'}=\frac{1}{2\sqrt{1+x^2}}\frac{d}{dx}\left(1+x^2\right)\end{align}

\begin{align}  y^{'}=\frac{2x}{2\sqrt{1+x^2}}\end{align}

\begin{align}  y^{'}=\frac{x}{\sqrt{1+x^2}}\end{align}

\begin{align}\Rightarrow y^{'}=\frac{x}{\sqrt{1+x^2}}\frac{\sqrt{1+x^2}}{\sqrt{1+x^2}}\end{align}

\begin{align}\Rightarrow y^{'}=\frac{x}{1+x^2}{\sqrt{1+x^2}}\end{align}

\begin{align}\Rightarrow y^{'}=\frac{x}{1+x^2}{y}\end{align}

\begin{align}\Rightarrow y^{'}=\frac{xy}{1+x^2}\end{align}

∴ L.H.S. = R.H.S.

Hence, the given function is the solution of the corresponding differential equation.

 

 


Q:
A:

y = Ax

Differentiating both sides with respect to x, we get: 

\begin{align}y^{'}=\frac{d}{dx}(Ax)\end{align}

⇒ y ' = A

Substituting the value of y' in the given differential equation, we get:

L.H.S. = xy= xA = Ax = y = R.H.S.

Hence, the given function is the solution of the corresponding differential equation.


Q:
A:

y= x.sinx

Differentiating both sides of this equation with respect to x, we get:

\begin{align} y^{'} =\frac{d}{dx}\left(x.sinx\right)\end{align}

\begin{align}\Rightarrow y^{'} =sinx. \frac{d}{dx}\left(x\right)+ x. \frac{d}{dx}\left(sinx\right)\end{align}

\begin{align} \Rightarrow y^{'} =sinx + x.cosx\end{align}

Differentiating both sides of this equation with respect to x, we get:

L.H.S. =xy= x(sinx + xcosx)

\begin{align} =x.sinx + x^2.cosx\end{align}

\begin{align} =y + x^2.\sqrt{1-sin^2x}\end{align}

\begin{align} =y + x^2.\sqrt{1-\left(\frac{y}{x}\right)^2}\end{align}

\begin{align} =y + x^2.\sqrt{\frac{x^2-y^2}{x^2}}\end{align}

\begin{align} =y + x.\sqrt{x^2-y^2}\end{align}

R.H.S.