Please reason step by step, and put your final answer within \boxed{}.
Given proposition $P$: If the radius of the inscribed circle of a triangle is $r$, and the lengths of its sides are $a$, $b$, and $c$, then the area of the triangle $S= \dfrac{1}{2}r(a+b+c)$. Try to, inspired by proposition $P$, propose a proposition $Q$ about a tetrahedron.
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