smc

To solve this problem, we could use the distance formula to find the lengths of the sides and then Heron's Formula to find the area of the triangle, but that solution seems messy and prone to mistakes with lots of square roots and polynomial expansions. Therefore, we look for a simpler, easier solution. Suppose $c=b$ and $a=d$. This makes $△PQR$ half of a rectangle with side lengths $a$ and $b$, so it has an area of $\frac{ab}{2}$. Plugging in $c=b$ and $d=a$ into all of the answer choices, the only one which returns $\frac{ab}{2}$ is $\boxed{\frac{ac+bd-ab}{2}}$.