Question 19
Circles $\omega_{1}$ and $\omega_{2}$ with centres $O_{1}$ and $O_{2}$ are externally tangent at point $D$ and internally tangent to a circle $\omega$ at points $E$ and $F$, respectively. Line $t$ is the common tangent of $\omega_{1}$ and $\omega_{2}$ at $D$. Let $AB$ be the diameter of $\omega$ perpendicular to $t$, so that $A$, $E$ and $O_{1}$ are on the same side of $t$. Prove that lines $A O_{1}$, $B O_{2}$, $EF$ and $t$ are concurrent.