How do you express cos(5π6)cos(5π12) without using products of trigonometric functions?

1 Answer
Mar 18, 2016

P=(34)(23)

Explanation:

P=cos(5π6).cos(5π12)
Trig table --> cos(5π6)=32
Find cos(5π12) by the identity: cos2a=2cos2a1
cos(5π6)=32=2cos2(5π12)1
2cos2(5π12)=132=232
cos2(5π12)=234
cos(5π12)=232 -->cos(5π12) is positive.
Finally,
P=(32)(232)=(34)(23)