Trigonomtery Angles: Compound Angle Formulas
What would be the formula for
-cos(A-B) = ?
-cos(A+B) = ?
-sin(A-B) = ?
-sin(A+B) = ?
I don't understand what happens with a negative in front of cos and sin, that's my point..
cos(11pi/12) ends up being -cos(pi/4-pi/6)
cos (17pi/12) ends up being -cos(pi/4-pi/6) also
[correct me if i am wrong]
and then, if i used the formula -cos(A-B) = -cosAcosB+sinAsinB
the formula on the right side of both questions are the same, yet if i leave the same formula on the right for both equations (since the left is the same) then i don't get the right answer.. one of them needs -cosAcosB-sinAsinB for the left and the other one needs -cosAcosB+sinAsinB in order to get the correct answer.
this is what I dont understand, how do you know which one it is?