1. The boundaries of the solids are surfaces.
2. The proved statements are called theorems.
3. If \(A\) is equal to \(B\) and \(B\) is equal to \(C\), then \(A\) \(=\) \(\frac{B}{2}\).
4. ‘For every line \(q\) and for every point \(O\) not lying on \(q\), there exists a unique line \(p\) passing through \(O\) and parallel to \(q\)' is called a Playfair's theorem.