\[a_{1}\ ;\ a_{1} + d;\ \ a_{1} + 2d;\ \ \]
\[a_{1} + 3d;\ \ldots\]
\[a_{1};\ \ a_{1} + 2d;\ \ a_{1} + 4d;\ldots\ \]
\[a_{1} + 2d = \frac{a_{1} + a_{1} + 4d}{2} =\]
\[= \frac{2a_{1} + 4d}{2} = a_{1} + 2d \Longrightarrow да,\]
\[будут.\]