Posted by lan on Sunday, May 8, 2011 at 9:56am.
cos 2A = 2cos^2 A - 1
cos150° = 2 cos^2 75 - 1
-√3/2 + 1 = 2cos^2 75°
cos^2 75° = (1 - √3/2)/2
cos 75° = √[(1 - √3/2)/2]
An easier ways would have been
cos75 = cos(45+30)
= cos45cos30 - sin45sin30
= (1/√2)(√3/2) - (1/√2)(1/2)
= (√3 - 1)/(2√2)
Use your calculator to show that both produce cos75
trig - Use a half-angle formula to find the exact value of cos 5ð/12
Trig - Use the half angle formula to find the exact value of tan 7 pie/8
trig - Use half-angle formulas to find the exact value for the following cot 75 ...
Trig - If cosx = 10/19 an pi < x < 2pi, find the exact value of ...
Trigonometry - use the appropriate trig identity (sum and difference, half angle...
trig - Find the exact value by using a half-angle identity. cos 22.5 degrees
MATH - 1.)Find the exact solution algebriacally, if possible: (PLEASE SHOW ALL ...
Trig - How do i find the exact value of sin 15 using the half angle formula
trig - using the fact that 5pi/8=1/2(5pi/8) and the half angle identity for cos ...
Trig - Use the half-angle formulas to come up with an exact expression for each ...
For Further Reading