Question:medium

If \[ \int \frac{dx}{\sin^3x+\cos^3x} = A\log\left| \frac{\sqrt2+t}{\sqrt2-t} \right| + B\tan^{-1}(t)+C, \] then \[ \left(\frac{B}{A},\,t\right) = \]

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For integrals containing \[ \sin^3x+\cos^3x, \] the substitution \[ t=\sin x-\cos x \] is highly effective because \[ (\sin x+\cos x)^2=2-t^2 \] and \[ \sin x\cos x=\frac{1-t^2}{2}. \]
Updated On: Jul 9, 2026
  • \[ \left(2\sqrt2,\;\sin x+\cos x\right) \]
  • \[ \left(\frac{\sqrt2}{9},\;\sin x+\cos x\right) \]
  • \[ \left(\frac{\sqrt2}{9},\;\sin x-\cos x\right) \]
  • \[ \left(2\sqrt2,\;\sin x-\cos x\right) \] 

Show Solution

The Correct Option is D

Solution and Explanation

Concept: Factor denominator using sum of cubes, substitute \(t = \sin x - \cos x\), express everything in t, use partial fractions, and integrate.

Step 1:
\(\sin^3x+\cos^3x = (\sin x+\cos x)(1-\sin x\cos x)\). Let \(t = \sin x - \cos x\). Then \(t^2 = 1 - 2\sin x\cos x \Rightarrow \sin x\cos x = (1-t^2)/2\). Also \((\sin x+\cos x)^2 = 1 + 2\sin x\cos x = 2-t^2\). \(dx = dt/(\cos x+\sin x) = dt/\sqrt{2-t^2}\).

Step 2:
Denominator = \(\sqrt{2-t^2} \cdot (1 - (1-t^2)/2) = \sqrt{2-t^2} \cdot (1+t^2)/2\). Integral: \(\int \frac{dx}{\sin^3x+\cos^3x} = \int \frac{dt/\sqrt{2-t^2}}{\sqrt{2-t^2}(1+t^2)/2} = \int \frac{2 dt}{(1+t^2)(2-t^2)}\).

Step 3:
Partial fractions: \(\frac{2}{(1+t^2)(2-t^2)} = \frac{2/3}{1+t^2} + \frac{2/3}{2-t^2}\). Integrate: \(\frac23\tan^{-1}t + \frac23 \cdot \frac{1}{2\sqrt2} \log\left|\frac{\sqrt2+t}{\sqrt2-t}\right| + C\). So \(A = \frac{1}{3\sqrt2}, B = \frac23\).

Step 4:
\(B/A = \frac{2/3}{1/(3\sqrt2)} = 2\sqrt2\). The argument is \(t = \sin x - \cos x\).

Step 5:
Write the final answer. \(\boxed{\left(2\sqrt2,\ \sin x-\cos x\right)}\)
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