April 2026

Prove x = (a−b)/(1+ab) If \( \sin^{-1}\left(\frac{2a}{1+a^2}\right) – \cos^{-1}\left(\frac{1-b^2}{1+b^2}\right) = \tan^{-1}\left(\frac{2x}{1-x^2}\right) \), prove that \( x = \frac{a-b}{1+ab} \) Solution: Use standard inverse trigonometric identities: \[ \sin^{-1}\left(\frac{2a}{1+a^2}\right) = 2\tan^{-1}(a) \] \[ \cos^{-1}\left(\frac{1-b^2}{1+b^2}\right) = 2\tan^{-1}(b) \] So the given equation becomes: \[ 2\tan^{-1}(a) – 2\tan^{-1}(b) = \tan^{-1}\left(\frac{2x}{1-x^2}\right) \] \[ \Rightarrow 2\left[\tan^{-1}(a) – \tan^{-1}(b)\right] = \tan^{-1}\left(\frac{2x}{1-x^2}\right) \] […]

Prove 4tan⁻¹(1/5) − tan⁻¹(1/239) = π/4 Prove that \( 4\tan^{-1}\left(\frac{1}{5}\right) – \tan^{-1}\left(\frac{1}{239}\right) = \frac{\pi}{4} \) Solution: Let \[ \theta = \tan^{-1}\left(\frac{1}{5}\right) \Rightarrow \tan \theta = \frac{1}{5} \] Step 1: Find \( \tan(2\theta) \) \[ \tan(2\theta) = \frac{2\tan\theta}{1 – \tan^2\theta} = \frac{2/5}{1 – 1/25} = \frac{2/5}{24/25} = \frac{5}{12} \] \[ \Rightarrow 2\theta = \tan^{-1}\left(\frac{5}{12}\right) \] Step

Prove 2tan⁻¹(1/2) + tan⁻¹(1/7) = tan⁻¹(31/17) Prove that \( 2\tan^{-1}\left(\frac{1}{2}\right) + \tan^{-1}\left(\frac{1}{7}\right) = \tan^{-1}\left(\frac{31}{17}\right) \) Solution: Let \[ \theta = \tan^{-1}\left(\frac{1}{2}\right) \] Then, \[ \tan \theta = \frac{1}{2} \] Using double angle identity: \[ \tan(2\theta) = \frac{2\tan\theta}{1 – \tan^2\theta} \] \[ = \frac{2 \cdot \frac{1}{2}}{1 – \frac{1}{4}} = \frac{1}{3/4} = \frac{4}{3} \] \[ \Rightarrow 2\theta

Prove 2tan⁻¹(3/4) − tan⁻¹(17/31) = π/4 Prove that \( 2\tan^{-1}\left(\frac{3}{4}\right) – \tan^{-1}\left(\frac{17}{31}\right) = \frac{\pi}{4} \) Solution: Let \[ \theta = \tan^{-1}\left(\frac{3}{4}\right) \] Then, \[ \tan \theta = \frac{3}{4} \] Using identity: \[ \tan(2\theta) = \frac{2\tan\theta}{1 – \tan^2\theta} \] \[ = \frac{2 \cdot \frac{3}{4}}{1 – \left(\frac{3}{4}\right)^2} = \frac{6/4}{1 – 9/16} = \frac{6/4}{7/16} = \frac{24}{7} \] \[

Prove 2tan⁻¹(1/5) + tan⁻¹(1/8) = tan⁻¹(4/7) Prove that \( 2\tan^{-1}\left(\frac{1}{5}\right) + \tan^{-1}\left(\frac{1}{8}\right) = \tan^{-1}\left(\frac{4}{7}\right) \) Solution: Let \[ \theta = \tan^{-1}\left(\frac{1}{5}\right) \] Then, \[ \tan \theta = \frac{1}{5} \] Using double angle identity: \[ \tan(2\theta) = \frac{2\tan\theta}{1 – \tan^2\theta} \] \[ = \frac{2 \cdot \frac{1}{5}}{1 – \frac{1}{25}} = \frac{2/5}{24/25} = \frac{5}{12} \] \[ \Rightarrow 2\theta

Prove 2sin⁻¹(3/5) − tan⁻¹(17/31) = π/4 Prove that \( 2\sin^{-1}\left(\frac{3}{5}\right) – \tan^{-1}\left(\frac{17}{31}\right) = \frac{\pi}{4} \) Solution: Let \[ \theta = \sin^{-1}\left(\frac{3}{5}\right) \Rightarrow \sin \theta = \frac{3}{5} \] Then, \[ \cos \theta = \frac{4}{5} \Rightarrow \tan \theta = \frac{3}{4} \] Using identity: \[ \tan(2\theta) = \frac{2\tan\theta}{1 – \tan^2\theta} \] \[ = \frac{2 \cdot \frac{3}{4}}{1 – \left(\frac{3}{4}\right)^2}

Prove sin⁻¹(4/5) + 2tan⁻¹(1/3) = π/2 Prove that \( \sin^{-1}\left(\frac{4}{5}\right) + 2\tan^{-1}\left(\frac{1}{3}\right) = \frac{\pi}{2} \) Solution: Let \[ \theta = \tan^{-1}\left(\frac{1}{3}\right) \] Then, \[ \tan \theta = \frac{1}{3} \] Using double angle identity: \[ \tan(2\theta) = \frac{2\tan\theta}{1 – \tan^2\theta} \] \[ = \frac{2 \cdot \frac{1}{3}}{1 – \frac{1}{9}} = \frac{2/3}{8/9} = \frac{3}{4} \] \[ \Rightarrow 2\theta

Prove tan⁻¹(1/7) + 2tan⁻¹(1/3) = π/4 Prove that \( \tan^{-1}\left(\frac{1}{7}\right) + 2\tan^{-1}\left(\frac{1}{3}\right) = \frac{\pi}{4} \) Solution: Let \[ \theta = \tan^{-1}\left(\frac{1}{3}\right) \] Then, \[ \tan \theta = \frac{1}{3} \] Using double angle identity: \[ \tan(2\theta) = \frac{2\tan\theta}{1 – \tan^2\theta} \] \[ = \frac{2 \cdot \frac{1}{3}}{1 – \left(\frac{1}{3}\right)^2} \] \[ = \frac{2/3}{1 – 1/9} = \frac{2/3}{8/9}

Prove tan⁻¹(2/3) = ½tan⁻¹(12/5) Prove that \( \tan^{-1}\left(\frac{2}{3}\right) = \frac{1}{2}\tan^{-1}\left(\frac{12}{5}\right) \) Solution: Let \[ \theta = \tan^{-1}\left(\frac{2}{3}\right) \] Then, \[ \tan \theta = \frac{2}{3} \] Using identity: \[ \tan(2\theta) = \frac{2\tan\theta}{1 – \tan^2\theta} \] \[ = \frac{2 \cdot \frac{2}{3}}{1 – \left(\frac{2}{3}\right)^2} \] \[ = \frac{4/3}{1 – 4/9} \] \[ = \frac{4/3}{5/9} \] \[ = \frac{4}{3}

Prove tan⁻¹(1/4) + tan⁻¹(2/9) = ½cos⁻¹(3/5) Prove that \( \tan^{-1}\left(\frac{1}{4}\right) + \tan^{-1}\left(\frac{2}{9}\right) = \frac{1}{2}\cos^{-1}\left(\frac{3}{5}\right) = \frac{1}{2}\sin^{-1}\left(\frac{4}{5}\right) \) Solution: Let \[ \theta = \tan^{-1}\left(\frac{1}{4}\right) + \tan^{-1}\left(\frac{2}{9}\right) \] Using identity: \[ \tan(A+B) = \frac{\tan A + \tan B}{1 – \tan A \tan B} \] \[ \tan \theta = \frac{\frac{1}{4} + \frac{2}{9}}{1 – \frac{1}{4}\cdot\frac{2}{9}} \] \[ = \frac{\frac{9