Related Formula
∫ (1)/(u² + a²) du = (1)/(a) ⁻¹((u)/(a)) + C$$\int \frac{1}{u^2 + a^2} du = \frac{1}{a} \tan^{-1}\left(\frac{u}{a}\right) + C$$
Core Logic
Simplify the given integrand by converting all trigonometric ratios into x$\sin x$ and x$\cos x$:
I = cosec x + xcosec x x + x ² x$$I = \frac{\operatorname{cosec} x + \sin x}{\operatorname{cosec} x \sec x + \tan x \sin^2 x}$$
Numerator: (1)/( x) + x = (1 + ² x)/( x)$\frac{1}{\sin x} + \sin x = \frac{1 + \sin^2 x}{\sin x}$
Denominator: (1)/( x x) + ( x)/( x) · ² x = (1 + ⁴ x)/( x x)$\frac{1}{\sin x \cos x} + \frac{\sin x}{\cos x} \cdot \sin^2 x = \frac{1 + \sin^4 x}{\sin x \cos x}$
Divide Numerator by Denominator:
I = ((1 + ² x)/( x))/((1 + ⁴ x)/( x x)) = ((1 + ² x) x)/(1 + ⁴ x)$$I = \frac{\frac{1 + \sin^2 x}{\sin x}}{\frac{1 + \sin^4 x}{\sin x \cos x}} = \frac{(1 + \sin^2 x)\cos x}{1 + \sin^4 x}$$
Step 1: Apply Substitution
Now rewrite the integral:
y(x) = ∫ ((1 + ² x) x)/(1 + ⁴ x) dx$$y(x) = \int \frac{(1 + \sin^2 x)\cos x}{1 + \sin^4 x} dx$$
Let x = t$\sin x = t$, then x dx = dt$\cos x \, dx = dt$. The integral becomes:
y(x) = ∫ (1 + t²)/(1 + t⁴) dt$$y(x) = \int \frac{1 + t^2}{1 + t^4} dt$$
Divide numerator and denominator by t²$t^2$:
y(x) = ∫ (1 + (1)/(t²))/(t² + (1)/(t²)) dt$$y(x) = \int \frac{1 + \frac{1}{t^2}}{t^2 + \frac{1}{t^2}} dt$$
We know that t² + (1)/(t²) = (t - (1)/(t))² + 2$t^2 + \frac{1}{t^2} = \left(t - \frac{1}{t}\right)^2 + 2$. Let u = t - (1)/(t)$u = t - \frac{1}{t}$, then du = (1 + (1)/(t²)) dt$du = \left(1 + \frac{1}{t^2}\right) dt$.
y(x) = ∫ duu² + (√(2))² = 1√(2) ⁻¹( u√(2)) + C$$y(x) = \int \frac{du}{u^2 + (\sqrt{2})^2} = \frac{1}{\sqrt{2}}\tan^{-1}\left(\frac{u}{\sqrt{2}}\right) + C$$
y(x) = 1√(2) ⁻¹( t - 1/t√(2)) + C = 1√(2) ⁻¹( x - cosec x√(2)) + C$$y(x) = \frac{1}{\sqrt{2}}\tan^{-1}\left(\frac{t - 1/t}{\sqrt{2}}\right) + C = \frac{1}{\sqrt{2}}\tan^{-1}\left(\frac{\sin x - \operatorname{cosec} x}{\sqrt{2}}\right) + C$$
Step 2: Solve for Constant C
We are given that xarrow(π/2)^- y(x) = 0$\lim_{x\rightarrow(\pi/2)^-} y(x) = 0$.
As x → (π)/(2)$x \to \frac{\pi}{2}$, x → 1$\sin x \to 1$ and cosec x → 1$\operatorname{cosec} x \to 1$.
Thus, x - cosec x → 0$\sin x - \operatorname{cosec} x \to 0$.
0 = 1√(2) ⁻¹( 1 - 1√(2)) + C$$0 = \frac{1}{\sqrt{2}}\tan^{-1}\left(\frac{1 - 1}{\sqrt{2}}\right) + C$$
0 = 1√(2) ⁻¹(0) + C ⇒ C = 0$$0 = \frac{1}{\sqrt{2}}\tan^{-1}(0) + C \Rightarrow C = 0$$
Step 3: Find y(pi/4)
Substitute x = (π)/(4)$x = \frac{\pi}{4}$ into the exact solution y(x) = 1√(2) ⁻¹( x - cosec x√(2))$y(x) = \frac{1}{\sqrt{2}}\tan^{-1}\left(\frac{\sin x - \operatorname{cosec} x}{\sqrt{2}}\right)$:
At x = (π)/(4)$x = \frac{\pi}{4}$, ((π)/(4)) = 1√(2)$\sin\left(\frac{\pi}{4}\right) = \frac{1}{\sqrt{2}}$ and cosec((π)/(4)) = √(2)$\operatorname{cosec}\left(\frac{\pi}{4}\right) = \sqrt{2}$.
y((π)/(4)) = 1√(2) ⁻¹( 1√(2) - √(2)√(2))$$y\left(\frac{\pi}{4}\right) = \frac{1}{\sqrt{2}}\tan^{-1}\left(\frac{\frac{1}{\sqrt{2}} - \sqrt{2}}{\sqrt{2}}\right)$$
= 1√(2) ⁻¹( 1 - 2√(2)√(2)) = 1√(2) ⁻¹( -1/√(2)√(2))$$= \frac{1}{\sqrt{2}}\tan^{-1}\left(\frac{\frac{1 - 2}{\sqrt{2}}}{\sqrt{2}}\right) = \frac{1}{\sqrt{2}}\tan^{-1}\left(\frac{-1/\sqrt{2}}{\sqrt{2}}\right)$$
= 1√(2) ⁻¹(-(1)/(2))$$= \frac{1}{\sqrt{2}}\tan^{-1}\left(-\frac{1}{2}\right)$$
Pattern Recognition
A high-degree polynomial of x$\sin x$ or x$\cos x$ in the denominator matched with their derivative elements on top is the hallmark of the t+1/t$t+1/t$ or t-1/t$t-1/t$ structural substitution form. Dividing out the middle power (t²$t^2$) aligns the differential instantly.
Chapter Mix
Class 12 Mathematics: Integral Calculus