Question
Write the value of $\cos^{-1}(\cos1540^\circ).$

Answer

We know that
$\cos^{-1}(\cos\text{x})=\text{x}$
Now,
$\cos^{-1}(\cos1540^\circ)=\cos^{-1}\{\cos(1440+100^\circ)\}$
$=\cos^{-1}\{\cos(100^\circ)\}$ $[\because\ \cos(4\pi+100^\circ)=\cos100^\circ]$
$=100^\circ$

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