Mathematics

Problem

\mathbf{r}_1, \mathbf{r}_2

and

\mathbf{r}_3

are the position vectors of three collinear points and scalars

l

and

m

exists such that

\mathbf{r}_3 = l\mathbf{r}_1 + m\mathbf{r}_2

, then show that

l + m = 1