Related Formula
d((y)/(x)) = (x dy - y dx)/(x²)$$d\left(\frac{y}{x}\right) = \frac{x \, dy - y \, dx}{x^2}$$
Core Logic
Rewrite the differential equation:
x dy - y dx = x² x dx$$x \, dy - y \, dx = x^2 \cot x \, dx$$
Divide by x²$x^2$ to create an exact differential:
(x dy - y dx)/(x²) = x dx$$\frac{x \, dy - y \, dx}{x^2} = \cot x \, dx$$
d((y)/(x)) = x dx$$d\left(\frac{y}{x}\right) = \cot x \, dx$$
Execution
Integrate both sides:
∫ d((y)/(x)) = ∫ x dx$$\int d\left(\frac{y}{x}\right) = \int \cot x \, dx$$
(y)/(x) = ln| x| + C$$\frac{y}{x} = \ln|\sin x| + C$$
Given y((π)/(2)) = (π)/(2)$y\left(\frac{\pi}{2}\right) = \frac{\pi}{2}$:
(π/2)/(π/2) = ln| (π)/(2)| + C ⇒ 1 = 0 + C ⇒ C = 1$$\frac{\pi/2}{\pi/2} = \ln|\sin\frac{\pi}{2}| + C \Rightarrow 1 = 0 + C \Rightarrow C = 1$$
Equation of curve: y = x(ln| x| + 1)$y = x(\ln|\sin x| + 1)$
Evaluate at limits:
y((π)/(6)) = (π)/(6)(ln(1)/(2) + 1) = (π)/(6)(-ln 2 + 1)$y\left(\frac{\pi}{6}\right) = \frac{\pi}{6}(\ln\frac{1}{2} + 1) = \frac{\pi}{6}(-\ln 2 + 1)$
y((π)/(4)) = (π)/(4)(ln 1√(2) + 1) = (π)/(4)(-(1)/(2)ln 2 + 1)$y\left(\frac{\pi}{4}\right) = \frac{\pi}{4}(\ln\frac{1}{\sqrt{2}} + 1) = \frac{\pi}{4}\left(-\frac{1}{2}\ln 2 + 1\right)$
Calculate 6y((π)/(6)) - 8y((π)/(4))$6y\left(\frac{\pi}{6}\right) - 8y\left(\frac{\pi}{4}\right)$:
= 6[(π)/(6)(-ln 2 + 1)] - 8[(π)/(4)(-(1)/(2)ln 2 + 1)]$$= 6\left[\frac{\pi}{6}(-\ln 2 + 1)\right] - 8\left[\frac{\pi}{4}\left(-\frac{1}{2}\ln 2 + 1\right)\right]$$
= π(-ln 2 + 1) - 2π(-(1)/(2)ln 2 + 1)$$= \pi(-\ln 2 + 1) - 2\pi\left(-\frac{1}{2}\ln 2 + 1\right)$$
= -πln 2 + π + πln 2 - 2π = -π$$= -\pi\ln 2 + \pi + \pi\ln 2 - 2\pi = -\pi$$
Pattern Recognition
Whenever you see x dy - y dx$x \, dy - y \, dx$, immediately test division by x²$x^2$, y²$y^2$, or xy$xy$ to convert it directly into exact differential forms like d(y/x)$d(y/x)$ or d(x/y)$d(x/y)$.
Chapter Mix
Class 12 Maths: Differential Equations