Please help i don’t get it 6th grade

If point cis between points a and b, then ac + — = ab

What is the domain of the following list of ordered pairs? (-2, 1), (2,0), (-1,-3), (4, 6)

Tangent ? ) × cos /2 +? ) > 0. the answer is that cos ? < 0. i forgot to put the minus before. i get confusermd when i see questions similar to this when the radian is negative such as /2 i cant seem to solve it