# Trigonometric Functions

**URL:** <https://forum.kirupa.com/t/trigonometric-functions/53400>\
**Category:** random\
**Created:** [June 3, 2004, 11:01am UTC](https://forum.kirupa.com/t/trigonometric-functions/53400 "2004-06-03T11:01:23Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![minimalistik](https://avatars.discourse-cdn.com/v4/letter/m/e47774/32.png) [@minimalistik](https://forum.kirupa.com/u/minimalistik)\
**Post date:** [June 3, 2004, 11:01am UTC](https://forum.kirupa.com/t/trigonometric-functions/53400/1 "2004-06-03T11:01:23Z")

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Hey again,

I have these three particular questions that I can’t get answer, i just float around it but never close to the bullseye.

1. sin (P + Q) cos (P + R) - cos (P + Q) sin (P + R)
2. cos (x + y) cos x + sin (x + y) . sin x
3. sin (A + B - C) cos (A - B + C) + cos (A + B - C) . sin (A - B + C)

the rules relating to this are:

-\> sin (a + b) = sina cosb + cosa sinb  
-\> sin (a - b) = sina cosb - cos a sinb  
-\> cos (a + b) = cosa cos b - sina sinb  
-\> cos (a - b) = cosa cosb + sina sinb

Do I collect the like terms and then expand them or do I multiply how it is, expand it and then collect like terms? Some help would be great.