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

Answer 1

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

Answer 2

uh..what?

Answer 3

should it be th edecimel or the other two ?

Answer 4

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

Answer 5

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

Answer 6

im confused.

Answer 7

anyways moving on ..next question

Answer 8

ty for your time.

Answer 9

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