CBSE 12th Standard Maths Subject Differential Equations Ncert Exemplar 4 Mark Questions With Solution 2021
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CBSE 12th Standard Maths Subject Differential Equations Ncert Exemplar 4 Mark Questions With Solution 2021
12th Standard CBSE
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Reg.No. :
Maths
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Solve the differential equation: y+ \(\frac {d}{dx}\)(xy) = s (sin x + log x)
(a) -
Find the general solution of the following differential equations :
\(\left( 1+{ y }^{ 2 } \right) +\left( x-{ e }^{ \tan ^{ -1 }{ y } } \right) \frac { dy }{ dx } =0\)(a) -
Find the differential equation of system of concentric circles with centre (1,2).
(a) -
Find the equation of a curve whose tangent at any point on it, different from origin, has slope \(y+\frac{y}{x}\).
(a) -
Find the equation of a curve passing through the point (1, 1), if the tangent drawn at any point P(x, y) on the curve meets the coordinate axes at A and B such that P is the mid-point of AB.
(a)
4 Marks
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CBSE 12th Standard Maths Subject Differential Equations Ncert Exemplar 4 Mark Questions With Solution 2021 Answer Keys
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x2y = -x2cos x + 2x sin x + 2 cos x + \(\frac { { x }^{ 3 } }{ 3 } logx-\frac { { x }^{ 3 } }{ 9 } +c\) is the required solution.
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Given, differential equation is \(\left(1+y^2\right)+\left(x-e^{\tan ^{-1} y}\right) \frac{d y}{d x}=0\)
It can be rewritten as
\(\left(1+y^2\right) \frac{d x}{d y}+x-e^{\tan ^{-1} y}=0\)
or \(\frac{d x}{d y}+\frac{1}{\left(1+y^2\right)} x=\frac{e^{\tan ^{-1} y}}{1+y^2}\)
[dividing both sides by (1 + y2)]
It is a linear differential equation of the form \(\frac{d x}{d y}+P x=Q\)
Here, \(P=\frac{1}{1+y^2} \text { and } Q=\frac{e^{\tan ^{-1} y}}{1+y^2}\)
Now, integrating factor, IF = \(e^{\int P d y}\)
\(=e^{\int \frac{1}{1+y^2} d y}=e^{\tan ^{-1} y}\)
\(\therefore\) The general solution of linear differential equation is given by
\(\begin{aligned} x \times \mathrm{IF} & =\int(Q \times \mathrm{IF}) d y+C \end{aligned}\)
\(\begin{aligned} \Rightarrow x \times e^{\tan ^{-1} y} & =\int \frac{e^{\tan ^{-1} y}}{1+y^2} \times e^{\tan ^{-1} y} d y+C \end{aligned}\)
\(\Rightarrow \quad x e^{\tan ^{-1} y}=\int \frac{e^{2 \tan ^{-1} y}}{1+y^2} d y+C\) ....(i)
On putting tan-1 y = t \(\Rightarrow \frac{1}{1+y^2} d y=d t \text { in }\)
Eq. (i), we get
\(\begin{aligned} x e^{\tan ^{-1} y} & =\int e^{2 t} d t+C \end{aligned}\)
\(\begin{aligned} \Rightarrow \quad x e^{\tan ^{-1} y} & =\frac{e^{2 t}}{2}+C \end{aligned}\)
\(\Rightarrow \quad x e^{\tan ^{-1} y}=\frac{e^{2 \tan ^{-1} y}}{2}+C \quad\left[\because t=\tan ^{-1} y\right]\) -
Equation of system of concentric circles with centre
\((1,2) \text { is }(x-1)^{2}+(y-2)^{2}=a^{2}\)
where a is radius of circle
\(=(x-1)+(y-2) \frac{d y}{d x}=0\) -
\(\frac{d y}{d x}=y+\frac{y}{x}\)
\(y=K x e^{x}\) -
The figure obtained by the given information is given below
Let the coordinate of the point P be (x, y). It is given that P is mid-point of AB. So, the coordinates of points A and are (2x, 0) and (0, 2y), respectively.
Now, slope of \(A B=\frac{0-2 y}{2 x-0}=-\frac{y}{x}\)
Since, the segment AB is a tangent to the curve at P.
\(\therefore \frac{d y}{d x}=-\frac{y}{x} \Rightarrow \frac{d y}{y}=-\frac{d x}{x}\)
On integrating both sides, we get
\( \log |y|=-\log |x|+\log |C| \)
\(\Rightarrow \log |y x|=\log |C| \)
\(\Rightarrow y x=C\)
Since, the given curve passes through the point (1, 1).
\(\therefore \ 1 \cdot 1=C \Rightarrow C=1\)
On putting C = 1in Eq. (i), we get
xy = 1
4 Marks