Answer::
[tex]\cos 75\degree=\frac{\sqrt[]{2-\sqrt[]{3}}}{2}[/tex]
Explanation:
By the half-angle formula:
[tex]\cos \mleft(\frac{\theta}{2}\mright)=\pm\sqrt[]{\frac{1+\cos\theta}{2}}[/tex]
Let θ=150°, therefore:
[tex]\begin{gathered} \cos (\frac{150\degree}{2})=\sqrt[]{\frac{1+\cos150\degree}{2}} \\ \cos (150)\degree=-\cos (180\degree-150\degree)=-\cos 30\degree \\ \implies\cos (\frac{150\degree}{2})=\sqrt[]{\frac{1+\cos150\degree}{2}}=\sqrt[]{\frac{1-\cos30\degree}{2}} \end{gathered}[/tex]
Now, cos 30 = √3/2, thus:
[tex]\begin{gathered} =\sqrt[]{\frac{1-\frac{\sqrt[]{3}}{2}}{2}} \\ \text{Multiply both the denominator and numerator by 2} \\ =\sqrt[]{\frac{2-\sqrt[]{3}}{4}} \\ =\frac{\sqrt{2-\sqrt[]{3}}}{\sqrt{4}} \end{gathered}[/tex]
The exact value of cos 75° is:
[tex]\cos 75\degree=\frac{\sqrt[]{2-\sqrt[]{3}}}{2}[/tex]
