"Constructiblelengths are those that can be built from known lengths by the operations of $\displaystyle +$, -, $\displaystyle \times$, $\displaystyle \%$ and $\displaystyle \sqrt$."

How do you go about it?Show, using similar triangles, that if lengths $\displaystyle l_1$ and $\displaystyle l_2$ areconstructible, then so are $\displaystyle l_1l_2$ and $\displaystyle l_1/l_2$.

How do you get a properminus signin $\displaystyle LaTeX$? I've tried "-", "$-$", "--", ... nothing works.