Welcome :: Homework Help and Answers :: Mathskey.com
Welcome to Mathskey.com Question & Answers Community. Ask any math/science homework question and receive answers from other members of the community.

13,435 questions

17,804 answers

1,438 comments

157,628 users

Find the exact value for cos 165° using the Half-Angle Identity.

0 votes
Have no clue how to do this.

 

asked Nov 26, 2013 in TRIGONOMETRY by linda Scholar

2 Answers

0 votes

Cos165 = Cos(180-15)

= -Cos15

= -Cos(30/2)

= -sqrt[(1+Cos30)/2]

= - sqrt[1+sqrt(3)/2]/2

= -sqrt[2+sqrt(3)/4]

= -sqrt[2+sqrt(3)]/2

answered Jan 21, 2014 by dozey Mentor
0 votes

image

The cos function is negative in second quadrant  and third quadrants.

In second quadrant cos (180 - 15) = - cos(15).

image

image

From the Double - Angle formulas, one may generate easily the Half - Angle formulas.

image

image.

image

image

image

image.

The exact value of  imageimage.

answered Jun 17, 2014 by lilly Expert
edited Jun 17, 2014 by lilly

Related questions

asked Apr 21, 2014 in TRIGONOMETRY by anonymous
...