It takes two points to define a straight line , three points to fully define a quadratic , four points to define a cubic curve , and so on . That is , it takes `` t '' points to define a polynomial of degree `` t-1 '' .
The idea is that a polynomial of degree t-1 is defined by t points on the polynomial : It takes two points to define a straight line , three to define a quadratic curve , four for a cubic , and so on .