Posted by **Elle** on Tuesday, September 27, 2011 at 12:21pm.

The identities cos(a-b)=cos(a)cos(b)sin(a)sin(b) and sin(a-b)=sin(a)cos(b)-cos(a)sin(b) are occasionally useful. Justify them. One method is to use rotation matricies. Another method is to use the established identities for cos(a+b) and sin (a+b).

- Trig! -
**Steve**, Tuesday, September 27, 2011 at 3:09pm
Sounds like a good justification to me. Oh, did you mean prove them? In that case, using the identities,

cos(a-b) = cos(a + (-b)) = cos(a) cos(-b) - sin(a) sin(-b)

= cos(a)cos(b) + sin(a) sin(b)

sin(a-b) = sin(a + (-b)) = sin(a) cos(-b) + cos(a) sin(-b)

= sin(a) cos(b) - cos(a) sin(b)

- Trig! -
**Bruce**, Sunday, November 6, 2011 at 3:13pm
Sin2x

## Answer This Question

## Related Questions

- 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 - Reduce the following to the sine or cosine of one angle: (i) sin145*cos75...
- tigonometry - expres the following as sums and differences of sines or cosines ...
- algebra - Can someone please help me do this problem? That would be great! ...
- Trig - Given: cos u = 3/5; 0 < u < pi/2 cos v = 5/13; 3pi/2 < v < ...
- Trig - Find sin(s+t) and (s-t) if cos(s)= 1/5 and sin(t) = 3/5 and s and t are ...
- Math(Please check) - Use the fundamental identities to simplify the expression. ...
- Precal - I do not understand how to do this problem ((sin^3 A + cos^3 A)/(sin A...
- trig - The expression 4 sin x cos x is equivalent to which of the following? (...

More Related Questions