Core Logic
(dy)/(dx) = (( x)/( x) + y)/( x ((1)/( x) - ( ² x)/( x))) = ( x + y x)/( x (1 - ² x))$$\frac{dy}{dx} = \frac{\frac{\sin x}{\cos x} + y}{\sin x \left(\frac{1}{\cos x} - \frac{\sin^2 x}{\cos x}\right)} = \frac{\sin x + y\cos x}{\sin x (1 - \sin^2 x)}$$
(dy)/(dx) = ( x + y x)/( x ² x) = ² x + (2y)/( 2x)$$\frac{dy}{dx} = \frac{\sin x + y\cos x}{\sin x \cos^2 x} = \sec^2 x + \frac{2y}{\sin 2x}$$
(dy)/(dx) - 2 (2x)y = ² x$$\frac{dy}{dx} - 2\csc(2x)y = \sec^2 x$$
This is an LDE of form (dy)/(dx) + Py = Q$\frac{dy}{dx} + Py = Q$.
I.F. = e∫ -2 (2x) dx$$I.F. = e^{\int -2\csc(2x) dx}$$
Let 2x = t 2dx = dt$2x = t \implies 2dx = dt$.
I.F. = e-∫ t dt = e-ln| (t/2)| = e-ln| x| = (1)/(| x|)$$I.F. = e^{-\int \csc t dt} = e^{-\ln|\tan(t/2)|} = e^{-\ln|\tan x|} = \frac{1}{|\tan x|}$$
Step 2: Solution of LDE
y(I.F.) = ∫ Q(I.F.) dx + C$$y(I.F.) = \int Q(I.F.) dx + C$$
y(1)/( x) = ∫ ² x (1)/( x) dx + C$$y\frac{1}{\tan x} = \int \sec^2 x \frac{1}{\tan x} dx + C$$
Let x = t ² x dx = dt$\tan x = t \implies \sec^2 x dx = dt$.
y(1)/( x) = ∫ (dt)/(t) + C = ln| x| + C$$y\frac{1}{\tan x} = \int \frac{dt}{t} + C = \ln|\tan x| + C$$
y = x(ln| x| + C)$$y = \tan x(\ln|\tan x| + C)$$
Step 3: Boundary Value
Given y(π/4) = 2$y(\pi/4) = 2$:
2 = 1(ln 1 + C) C = 2$$2 = 1(\ln 1 + C) \implies C = 2$$
Thus, y = x (ln| x| + 2)$y = \tan x (\ln|\tan x| + 2)$.
At x = π/3$x = \pi/3$:
y(π/3) = √(3)(ln√(3) + 2)$$y(\pi/3) = \sqrt{3}(\ln\sqrt{3} + 2)$$
Chapter Mix
Class 12 Maths: Differential Equations