Question

What's the formula to prove 3 points a collinear?

Answer 1

Find the equation of the line through any two of the points. Show the third point satisfies the equation.

Answer 2

1) Get the distance between Point A and Point B. That's Line AB
2) Get the distance between Point B and Point C. That's Line BC
3) Add Line AB and Line BC. They must be equal to Line AC.

another way is to see if they are proportional