The table shown weighs $30 \mathrm{lb}$ and has a diameter of $4 \mathrm{ft}$. It is supported by three legs equally spaced around the edge. A vertical load $\mathrm{P}$ with a magnitude of $100 \mathrm{lb}$ is applied to the top of the table at $D .$ Determine the maximum value of $a$ if the table is not to tip over. Show, on a sketch, the area of the table over which $\mathrm{P}$ can act without tipping the table.