Posted by **Hannah** on Wednesday, March 17, 2010 at 11:25pm.

use the sum and difference identities to find the cosine angle

cos pi/9 cos pi/3 - sin pi/9 sin pi/3

I do not know how to solve this because pi/9 is not on the unit circle.

- Math -
**Reiny**, Wednesday, March 17, 2010 at 11:35pm
but it gave you a hint, ...

cos(A+B) = cosAcosB - sinAsinB

does the right side not follow the pattern of your question?

so ...

cos pi/9 cos pi/3 - sin pi/9 sin pi/3

= cos(pi/9 + pi/3)

= cos(4pi/9)

You are right, 4pi/9 radians, or 80 degrees, is not one of the standard angles on the unit circle, nor is it obtainable using combinations of our standard angles.

## Answer This Question

## Related Questions

- trig - Reduce the following to the sine or cosine of one angle: (i) sin145*cos75...
- Mathematics - Trigonometric Identities - Let y represent theta Prove: 1 + 1/tan^...
- TRIG! - Posted by hayden on Monday, February 23, 2009 at 4:05pm. sin^6 x + cos^6...
- Trig - Given: cos u = 3/5; 0 < u < pi/2 cos v = 5/13; 3pi/2 < v < ...
- MATH - 1.)Find the exact solution algebriacally, if possible: (PLEASE SHOW ALL ...
- Trig! - The identities cos(a-b)=cos(a)cos(b)sin(a)sin(b) and sin(a-b)=sin(a)cos(...
- algebra - Can someone please help me do this problem? That would be great! ...
- tigonometry - expres the following as sums and differences of sines or cosines ...
- Math(Please check) - Use the fundamental identities to simplify the expression. ...
- Math(Please help) - 1)tan Q = -3/4 Find cosQ -3^2 + 4^2 = x^2 9+16 = sqrt 25 = 5...

More Related Questions