How too multiply 2 rotational matrices like this:
R(THETA2) . R(THETA1) = R(THETA1 +THETA2)
How would that look given the rotation matrix:
cos(THETA) -sin(THETA)
sin(THETA) COS(THETA)
It would be very useful to memorise them; they seem to crop up all the time.
There is a nice proof using Euler's formula, if you've ever come across that. Investigate more here Trigonometric Addition Formulas -- from Wolfram MathWorld