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Find the value of tan-1 [tan (5π / 6)] + cos-1 [cos (13π / 6)].

Consider \(\begin{array}{l}\tan \left(\frac{5 \pi}{6}\right)=\tan \left(\pi-\frac{\pi}{6}\right) \\ =-\tan \frac{\pi}{6} \\ =-\frac{1}{\sqrt{3}} \\ \cos \left(13 \frac{\pi}{6}\right)=\cos \left(2 \pi+\frac{\pi}{6}\right) \\ =\tan ^{-1}\left(\frac{-1}{\sqrt{3}}\right)+\cos ^{-1}\left(\frac{\sqrt{3}}{2}\right)... View Article

nPr and nCr are equal when

They are equal when \(\begin{array}{l}^{\mathrm{n}} \mathrm{P}_{\mathrm{r}}={ }^{\mathrm{n}} \mathrm{C}_{\mathrm{r}} \\ \frac{\mathrm{n} !}{(\mathrm{n}-\mathrm{r}) !}=\frac{\mathrm{n} !}{(\mathrm{n}-\mathrm{r}) ! \cdot \mathrm{r} !} \\ \mathrm{r} !=1... View Article