The given differential equation is $$y = x\frac{{dy}}{{dx}} + \frac{{dx}}{{dy}}$$
Multiplying both the sides by $$\frac{{dy}}{{dx}}$$ we get
$$\eqalign{
& \left( {\frac{{dy}}{{dx}}} \right)y = x{\left( {\frac{{dy}}{{dx}}} \right)^2} + 1 \cr
& \Rightarrow x{\left( {\frac{{dy}}{{dx}}} \right)^2} - y\left( {\frac{{dy}}{{dx}}} \right) + 1 = 0 \cr} $$
Hence, order and degree of differential equation are $$1$$ and $$2.$$
2.
A curve is such that the portion of the $$x$$-axis cut off between the origin and the tangent at a point is twice the abscissa and which passes through the point $$\left( {1,\,2} \right).$$ The equation of the curve is :
Let $$P\left( {x,\,y} \right)$$ be any point on the curve, $$PM$$ the perpendicular to $$x$$-axis $$PT$$ the tangent at $$P$$ meeting the axis of $$x$$ at $$T.$$ As given $$OT = 2\,OM =2x.$$ Equation of the tangent at $$P\left( {x,\,y} \right)$$ is $$Y - y = \frac{{dy}}{{dx}}\left( {X - x} \right)$$
It intersects the axis of $$x$$ where $$Y = 0$$
i.e., $$ - y = \frac{{dy}}{{dx}}\left( {X - x} \right){\text{ or }}X = x - y\frac{{dy}}{{dx}} = OT$$
Hence, $$x - y\frac{{dy}}{{dx}} = 2x{\text{ or }}\frac{{dx}}{x} + \frac{{dy}}{y} = 0$$
Integrating, $$\log \,x + \log \,y = \log \,C{\text{ i}}{\text{.e}}{\text{., }}xy = C$$
This passes through $$\left( {1,\,2} \right),$$
$$\therefore \,C = 2$$
Hence the required curve is $$xy = 2$$
3.
The differential equation of the family of circles with fixed radius $$5$$ units and centre on the line $$y = 2$$ is :
5.
A function $$y = f\left( x \right)$$ satisfies the differential equation $$\frac{{dy}}{{dx}} - y = \cos \,x - \sin \,x$$ with initial condition that $$y$$ is bounded when $$x \to \infty .$$ The area enclosed by $$y = f\left( x \right),\,y = \cos \,x$$ and the $$y$$-axis is :
Since order of the highest derivative in the given differential equation is $$1$$ and exponent of the derivative is also $$1$$ therefore degree and order is $$\left( {1,\,1} \right).$$
9.
If $$y = y\left( x \right)$$ and it follows the relation $$x\,\cos \,y + y\,\cos \,x = \pi $$ then $$y''\left( 0 \right) = $$