Question

Find the value of ${\cos }^2{12}^{\circ }+{\cos }^2{78}^{\circ }$

Answer 1

${\cos }^2{12}^o+{\cos }^2{78}^o$

$={\cos }^2({90}^o-{78}^o)+{\cos }^2{78}^o$

$={\sin }^2{78}^o+{\cos }^2{78}^o$ ..... $[\because \sin \theta =\cos ({90}^o-\theta \left)\right]$

$=1$ ...... $[\because {\sin }^2\theta +{\cos }^2\theta =1]$