Just like the answer downstairs: draw a solid line for the visible and a dotted line for the invisible. For example: draw a cube, or some 3D graphics (there is always a hidden side, so if you want to express its outline, use a dotted line).

The auxiliary line must be drawn as a dotted line. As the name implies, the auxiliary line is a line segment used to help you solve problems. Since you don't know this term, it means that you haven't been exposed to this knowledge. Learn slowly, and you will learn. For example, when doing an application problem, some problems give pictures, but you can't work out the answer just by the pictures it gives you, so you need to use auxiliary lines. (The auxiliary line primary school will mention and teach how to use it, and the junior high school will study it in detail.)

There are also requirements in algebra, rarely, hardly. Comparatively speaking, geometry is the most.

If you can't make it clear in a few words, you have to give an example to see the details.