\hsize 0pt\vsize 0pt\nopagenumbers\overfullrule 0pt\noindent$$\eqalign{y &= (1 - \alpha)x_1 + \alpha x_2 \cr  &= x_1 - \alpha x_1 + \alpha x_2 \cr  &= x_1 + \alpha x_2 - \alpha x_1 \cr  &= x_1 + (x_2 - x_1)\alpha}$$\bye