Hello laurettarosa! Are you familiar with the C.A.S.T Rule? Basically, it helps us determine where the trig ratios: sin, cos, and tan are positive or negative.
Let Q = theta.
The C in CAST is located in the 4th quadrant, and only Cosine is positive there.
The A in CAST is located in the 1st quadrant and Sine, Cosine, and Tan are positive there.
The S in CAST is located in the 2nd quadrant and only Sine is positive there
The T is CAST is located in the 3rd quadrant and only Tan is positive there.
cos(180 - Q) : Analysing the angle: 180 - Q, this means that Q is located in the 2nd quadrant and according to CAST, only sine is positive there.
Therefore, cos(180 - Q) = - cos Q
Can you handle the rest? if you have any other questions, please do not hesitate to ask.