The exact value of cos 5pi/ 12 is
A. sqrt2 (sqrt(3) + 1) / 4
b. sqrt2 (sqrt(3) - 1) / 4
c. .26
d. .99

Question

The exact value of cos 5pi/ 12 is 
A. sqrt2 (sqrt(3) + 1)   / 4 
b. sqrt2 (sqrt(3) - 1)   / 4
c.   .26
d. .99

raden · Answer

5pi/12 = 75 degrees, right?
cos75 = cos(45+30)
use the formula : cos(a+b) = cosacosb - sinasinb

anonymous · Answer

uh..what?

anonymous · Answer

should it be th edecimel or the other two ?

raden · Answer

just plug a=45 and b=30 into that formula, next simplify it

anonymous · Answer

i got .25888 which would have been c....but its not ...its b ...ty

anonymous · Answer

im confused.

anonymous · Answer

anyways moving on ..next question

anonymous · Answer

ty for your time.

raden · Answer

cos75 = cos(45+30)
= cos45cos30 - sin45sin30
= (1/2)sqrt(2)*(1/2)sqrt(3) - (1/2)sqrt(2)*(1/2)
= (1/4)sqrt(6) - (1/4)sqrt(2)
= (sqrt(6) - sqrt(2))/4
= sqrt(2){sqrt(3) - 1} / 4
B