# pre-calculus

posted by lina

use a sum or difference formula to find the exact of the trigonometric function
cos(-13pi/12)

1. Steve

cos(-13pi/12)
= cos(-13pi/12 + 24pi/12)
= cos(11pi/12)
= -cos(pi - 11pi/12)
= -cos(pi/12)

now, cos(x/2) = √((1+cos(x))/2)

-cos(pi/12) = -√((1+cos(pi/6))/2)
= -√((1+√3/2)/2)
= -√(2 + √3/4)
= -1/2 √(2+√3)

or
-1/2√2 (1+√3)

## Similar Questions

1. ### trig

use sum or difference identities to find the exact value of the trigonometric function: tan 345 please show work, i cant figure out what im doing wrong!
2. ### trig

determine an exact expression for the trigonometric function sin(13pi/12).
3. ### Math

Use the sum or difference identities to find the exact value of each trigonometric function. sec pi/12 Thanks :)
4. ### Math

Given that cos 13pi/18=sin y, first express 13pi/18 as a sum of pi/2 and an angle, and then apply a trigonometric identitiy to determine the measure of angle y.
5. ### TRIG/ALGEBRA

1) Find the exact value. Use a sum or difference identity. tan (-15 degrees) 2) Rewrite the following expression as a trigonometric function of a single angle measure. cos 3x cos 4x - sin 3x sine 4x
6. ### Precalculus/Trig 6

Find the exact value of the following trigonometric functions: sin(13pi), cos(14pi), tan(15pi)
7. ### Trigonometry

If cos(a)=1/2 and sin(b)=2/3, find sin(a+b), if 1) Both angles are acute; Answer: (sqrt(15)+2)/6 ii) a is an acute angle and pi/2 < b < pi; Answer: (2-sqrt(15))/6 2. Find the exact value of the six trigonometric functions of …
8. ### Homework Help Calculus

Find the linear approximation L(x)of the function f(x)=cos(pi/(6)x) at the point x=1 and use it to estimate the value of cos(13pi/72). Here's what I did so far: L(x)=sqrt(3)/2-1/12pi(x-1)+0((x-1)^2) How do I find cos(13pi/72)
9. ### Pre-Calc

1) Use an Addition or Subtraction Formula to write the expression as a trigonometric function of one number. cos 12° cos 18° − sin 12° sin 18° And Find its exact value. 2) Use an Addition or Subtraction Formula to write the …
10. ### Pre-Calc

I am really struggling with how to do these problems, I posted them a few minutes ago but the answers/work shown was incorrect. 1) a) Use an Addition or Subtraction Formula to write the expression as a trigonometric function of one …

More Similar Questions