KöMaL 2010 B4266

Let $a_1$, $a_2$, $a_3$, $a_4$ denote four consecutive elements in a row of Pascal's triangle. Prove that the numbers $\frac{a_{1}}{a_{1}+a_{2}}$, $\frac{a_{2}}{a_{2}+a_{3}}$, $\frac{a_{3}}{a_{3}+a_{4}}$ form an arithmetic progression.