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..

For example:

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?