\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