1. The problem statement, all variables and given/known data 1. Given cos α = -24/25 in Quadrant II and sin β = -5/13 in Quadrant III, find sin (α + β). 2. Given sin α = 4/5 in Quadrant I and cos β = -12/13 in Quadrant II, find cos (α + β). 2. Relevant equations I don't even know where to start! The textbook doesn't list any equations. 3. The attempt at a solution I have none, other than guessing that you multiply the denominators in each problem... other than that, I don't even know how to solve these types of problems. I'm without a teacher, so I'd really like an explanation since I'm a little... really... lost about these problems. Thanks to anyone who can show me how I'd go about solving these! I need to know it for an upcoming final.