Prove that tan−1(21)+tan−1(51)+tan−1(81)=4π.Answer: Proved: the sum equals 4π.SolutionAdd the first two:tan−121+tan−151=tan−11−10121+51=tan−19/107/10=tan−197Now add the third:tan−197+tan−181=tan−11−97⋅8197+81=tan−17272−77256+9=tan−16565=tan−11=4π4πCBSE Class 12 Board · 2011 · Subjective