1964 IMO Problems/Problem 6

Problem

In tetrahedron $ABCD$ , vertex $D$ is connected with $D_0$ , the centrod of $\triangle ABC$ . Lines parallel to $DD_0$ are drawn through $A,B$ and $C$ . These lines intersect the planes $BCD, CAD$ and $ABD$ in points $A_1, B_1,$ and $C_1$ , respectively. Prove that the volume of $ABCD$ is one third the volume of $A_1B_1C_1D_0$ . Is the result true if point $D_o$ is selected anywhere within $\triangle ABC$ ?

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