if the points (a,b) (a',b') and (a-a')(b-b')are collinear show that ab'=a'b
Here is the link for the answer similar to your query:
https://www.meritnation.com/ask-answer/question/p-q-m-n-p-m-q-n-are-collinear-show-that-p/coordinate-geometry/1702250
https://www.meritnation.com/ask-answer/question/p-q-m-n-p-m-q-n-are-collinear-show-that-p/coordinate-geometry/1702250