# Differential Equations

 Question 1
Which one of the following options contains two solutions of the differential equation $\frac{dy}{dx}=(y-1)x$?
 A $ln |y-1|=0.5x^2 +C$ and y=1 B $ln |y-1|=2x^2 +C$ and y=1 C $ln |y-1|=0.5x^2 +C$ and y=-1 D $ln |y-1|=2x^2 +C$ and y=-1
GATE EC 2020   Engineering Mathematics
Question 1 Explanation:
\begin{aligned} \frac{dy}{dx}&=(y-1)x\\ \frac{dy}{y-1}&=x dx\\ \text{Integrating}&\text{ both side, we get}\\ \int \frac{1}{y-1}dy&=\int xdx\\ \ln |y-1|&=\frac{x^2}{2}+C\; \; \;...(i)\\ \text{Now, }&\\ \frac{dy}{dx}&=0 \text{ when y=1}\\ \text{It means }y&= \text{constant C}\\ \therefore \; y&=1\;\;\;...(ii) \end{aligned}
So, equation (i) and (ii) are two different solution of given differential equation.
 Question 2
The general solution of $\frac{d^2y}{dx^2}-6\frac{dy}{dx}+9y=0$ is
 A $y=C_1e^{3x}+C_2e^{-3x}$ B $y=(C_1+C_2x)e^{-3x}$ C $y=(C_1+C_2x)e^{3x}$ D $y=C_1e^{3x}$
GATE EC 2020   Engineering Mathematics
Question 2 Explanation:
Taking $\frac{\mathrm{d} }{\mathrm{d} x}=D$
Given, $D^{2}-6D+9=0$
$(D-3)^2=0$
$D=3,3$
So, Solution of the given Differential equation
$y=(C_{1}+C_{2}x)e^{3x}$
 Question 3
Consider the homogeneous ordinary differential equation

$x^2\frac{d^2y}{dx^2}-3x\frac{dy}{dx}+3y=0, \; x \gt 0$

with y(x) as a generalsolution. Given that
y(1)=1 and y(2)=14
the value of y(1.5), rounded off to two decimal places, is ______.
 A 1.25 B 4.25 C 3.75 D 5.25
GATE EC 2019   Engineering Mathematics
Question 3 Explanation:
\begin{aligned} \left(x^{2} D^{2}-3 x D+3\right)&=0 \\ (0(0-1)-30+3) y&=0 \\ \left(0^{2}-40+3\right) y&=0 \\ \text { AE is }\quad m^{2}-4 m+3&=0 \\ m&=1.3 \\ CF&=C_{1}x+C_{2}x^{3} \\ \text { Solution is } \quad y&=C_{1}x+C_{2}x^{3} \\ y(1)=1\quad 1&=C_{1}+C_{2} \\ y(2)=14\quad 14&=2 C_{1}+8 C_{2} \\ \text{From (il) and (ii). }C_{1}&=-1, \quad C_{2}=2 \\ \therefore\quad y&=-x+2 x^{3} \\ y(1.5)&=-1.5+2(1.5)^{3}=5.25 \end{aligned}
 Question 4
The families of curves represented by the solution of the equation

$\frac{dy}{dx}=-\left (\frac{x}{y} \right )^n$

for $n = -1$ and $n = +1$, respectively, are
 A Parabolas and Circles B Circles and Hyperbolas C Hyperbolas and Circles D Hyperbolas and Parabolas
GATE EC 2019   Engineering Mathematics
Question 4 Explanation:
\begin{aligned} \frac{d y}{d x} &=-\left(\frac{x}{y}\right)^{n} \\ n=-1\quad\quad \frac{d y}{d x} &=-\frac{y^{\prime}}{x} \\ \frac{d y}{y} &=-\frac{d x}{x} \\ \int \frac{1}{y} d y &=-\int \frac{1}{x} d x \\ \ln y &=-\ln x+\ln c \\ \ln (y x) &=\ln c \end{aligned}
$x y=c \quad$ (Represents rectangular hyporbola)
\begin{aligned} n=1, \quad \frac{d y}{d x}&=-\frac{x}{y} \\ y d y &=-x d x \\ y d y &=-\int x d x \\ \frac{y^{2}}{2} &=-\frac{x^{2}}{2}+c \end{aligned}
$x^{2}+y^{2}=2 c \quad$ (Represents family of circles)
 Question 5
The position of a particle y(t) is described by the differential equation:

$\frac{d^{2}y}{dt^{2}}=-\frac{dy}{dt}-\frac{5y}{4}$

The initial conditions are y(0) = 1 and $\left.\begin{matrix} \frac{dy}{dt} \end{matrix}\right|_{t=0}=0$ . The position (accurate to two decimal places) of the particle at $t = \pi$ is _______.
 A -0.21 B -0.5 C -0.01 D -0.9
GATE EC 2018   Engineering Mathematics
Question 5 Explanation:
\begin{aligned} \frac{d^{2}}{d t^{2}}+\frac{dy}{dt}+\frac{5Y}{4}&=0\\ y(0)&=1 \\ y^{\prime}(0)&=0 \end{aligned}
By applying Laplace transform,
\begin{aligned} s^{2} Y(s)-s(1)+s Y(s)-1+\frac{5}{4} Y(s)=0 & \\ Y(s)=\frac{s+1}{s^{2}+s+\frac{5}{4}}=\frac{s+1}{\left(s+\frac{1}{2}\right)^{2}+1} \\ =\frac{\left(s+\frac{1}{2}\right)}{\left(s+\frac{1}{2}\right)^{2}+1}+\frac{\frac{1}{2}}{\left(s+\frac{1}{2}\right)^{2}+1} \end{aligned}
By taking inverse Laplace transform.
\begin{aligned} y(t)&=e^{-t / 2}\left[\cos (t)+\frac{1}{2} \sin (t)\right] ; t>0\\ \text{At }t=\pi &\\ y(t=\pi) &=e^{-\pi / 2}[(-1)+(0)] \\ &=-e^{-\pi / 2}=-0.2078 \simeq-0.21 \end{aligned}
 Question 6
The general solution of the differential equation $\frac{d^{2}y}{dx^{2}}+2\frac{dy}{dx}-5y=0$ in terms of arbitrary constants $K_{1} \; and \; K_{2}$ is
 A $K_{1}e^{(-1+\sqrt{6})x}+K_{2}e^{(-1-\sqrt{6})x}$ B $K_{1}e^{(-1+\sqrt{8})x}+K_{2}e^{(-1-\sqrt{8})x}$ C $K_{1}e^{(-2+\sqrt{6})x}+K_{2}e^{(-2-\sqrt{6})x}$ D $K_{1}e^{(-2+\sqrt{8})x}+K_{2}e^{(-2-\sqrt{8})x}$
GATE EC 2017-SET-2   Engineering Mathematics
Question 6 Explanation:
The general solution of the differential equation
\begin{aligned} \frac{d^{2} y}{d x^{2}}+2 \frac{d y}{d x}-5 y&=0 \\ \left(D^{2}+2 D-5\right) y&=0 \end{aligned}
Auxillary equation is
\begin{aligned} m^{2}+2 m-5 &=0 \\ m &=\frac{-2 \pm \sqrt{4+20}}{2}=\frac{-2 \pm \sqrt{24}}{2} \\ &=\frac{-2 \pm 2 \sqrt{6}}{2}=-1 \pm \sqrt{6} \\ \text { Solution is } \quad y &=K_{1} e^{(-1+\sqrt{6}) x}+K_{2} e^{(-1-\sqrt{6}) x} \end{aligned}
 Question 7
Which one of the following is the general solution of the first order differential equation $\frac{dy}{dx}=(x+y-1)^{2}$ , where x, y are real?
 A $y=1+x+tan^{-1}(x+c)$, where c is a constant B $y=1+x+tan(x+c)$, where c is a constant C $y=1-x+tan^{-1}(x+c)$, where c is a constant D $y=1-x+tan(x+c)$, where c is a constant
GATE EC 2017-SET-1   Engineering Mathematics
Question 7 Explanation:
\begin{aligned} \frac{d y}{d x}&=(x+y-1)^{2} &\ldots(i)\\ \text{Let, }x+y-1&=t &\ldots(ii)\\ 1+\frac{d y}{d x} &=\frac{d t}{d x} \\ \frac{d y}{d x} &=\frac{d t}{d x}-1 &\ldots(iii) \end{aligned}
Substituting equations (ii) and (iii) in equation (i)
\begin{aligned} \frac{d t}{d x}-1&=t^{2} \\ \frac{d t}{t^{2}+1}&=d x \end{aligned}
Integrating both side
\begin{aligned} \int \frac{1}{t^{2}+1} d t &=\int d x \\ \tan ^{-1} t &=x+c\\ \text{Since, }\quad\quad t&=x+y-1 \\ \therefore \tan ^{-1}(x+y-1)&=x+c \\ x+y-1&=\tan (x+c) \\ y&=1-x+\tan (x+c) \end{aligned}
 Question 8
The particular solution of the initial value problem given below is
$\frac{d^{2}y}{dx^{2}}+12\frac{dy}{dx}+36y=0$ with $y(0)=3$ and $\frac{dy}{dx}|_{x=0}=-36$
 A $(3-18x)e^{-6x}$ B $(3+25x)e^{-6x}$ C $(3+20x)e^{-6x}$ D $(3-12x)e^{-6x}$
GATE EC 2016-SET-3   Engineering Mathematics
Question 8 Explanation:
\begin{aligned} \left(D^{2}+12 D+36\right) &=0 \\ (D+6)^{2} y &=0 \\ D &=-6,-6\\ y &=\left(C_{1}+x C_{2}\right) e^{6 x} \\ y &=C_{1} e^{-6 x}+C_{2} x e^{-6 x} \\ y(0) &=3 \\ &=C_{1}+0 \\ \Rightarrow \quad C_{1} &=3 \\ y^{\prime} &=-6 C_{1} e^{6 x}+C_{2} \sigma^{6 x}-6 C_{2} x \theta^{6} x \\ y(0) &=-36 \\ -36 &=-6 C_{1}+C_{2} \\ -36 &=-18+C_{2} \\ C_{2} &=-18 \\ \therefore \quad \quad y &=3 e^{-6 x}-18 x \mathrm{e}^{-6 x} \\ y &=(3-18 x) e^{-6 x} \end{aligned}
 Question 9
Consider the differential equation
$\frac{d^{2}x(t)}{dt^{2}}+3\frac{dx(t)}{dt}+2x(t)=0$.
Given x(0) = 20 and x(1) = 10/e, where e = 2.718, the value of x(2) is _________.
 A 0.25 B 0.45 C 0.85 D 1
GATE EC 2015-SET-3   Engineering Mathematics
Question 9 Explanation:
\begin{aligned} D^{2}+3 D+2 &=0 \\ D &=-1,-2 \\ x(t) &=C_{1} e^{-t}+C_{2} e^{-2 t} \\ x(1) &=\frac{10}{e}=C_{1} e^{-1}+C_{2} e^{-2} \\ C_{1}+C_{2} e^{-1} &=10 \; \; \ldots(i)\\ C_{1}+C_{2} &=20 \; \; \ldots(ii)\\ \text { From here, } \quad C_{1}=& \cdot \frac{10 e-20}{e-1} ; \quad C_{2}=\left(\frac{10 e}{e-1}\right) \\ x(2) &=\left(\frac{10 e-20}{e-1}\right) e^{-2}+\left(\frac{10 e }{e-1}\right) e^{-4} \\ &=0.8566 \end{aligned}
 Question 10
The general solution of the differential equation $\frac{dy}{dx}=\frac{1+cos2y}{1-cos2x}$ is
 A tan y - cot x = c ( c is a constant) B tan x - cot y = c ( c is a constant) C tan y + cot x = c ( c is a constant) D tan x + cot y = c ( c is a constant)
GATE EC 2015-SET-2   Engineering Mathematics
Question 10 Explanation:
\begin{aligned} \frac{d y}{1+\cos 2 y}&=\frac{d x}{1-\cos 2 x} \\ \frac{d y}{2 \cos ^{2} y}&=\frac{d x}{2 \sin ^{2} x} \\ \sec ^{2} y d y&=\text{cosec}^{2} x d x\\ \end{aligned}
Integrating both sides, we get
\begin{aligned} \tan y&=-\cot x+c\\ \tan y+\text{cot} x&=c \end{aligned}
There are 10 questions to complete.