Question

The exact value of cos5pi/12 is:​

Answer 1

Step 1: Convert $\frac{5\pi }{12}$, which is in radians into degrees. To convert it multiply by $\frac{180}{\pi }$

$\frac{5\pi }{12} =\frac{180}{\pi }$

900/12

75

Step 2: 75 degrees isn't on the unit circle but 45 degrees and 30 degrees is. Since 45 + 30 = 75 you can use the cosine of 45 and 30 to find the exact value

cos45 = $\frac{\sqrt{2} }{2}$

cos30 =$\frac{\sqrt{3} }{2}$

Step 3: Add the cos45 and cos30 to get cos5pi/12

$\frac{\sqrt{2} }{2} +\frac{\sqrt{3} }{2} = \frac{\sqrt{2}+\sqrt{3}}{2}$

Hope this helped!