Rubber-backed protractors are worth it.

A protractor with a rubberized or textured back won’t slide on your math book. Most sliding comes from smooth plastic against smooth paper.

When you’re picking one out, flip it over and look at the underside. The good ones have a soft strip of silicone or a band of rubber around the edge. Some have little grippy nubs. If it’s all rigid, shiny plastic, it’s going to drift the second you try to draw the line.

The other thing to check is size. A slightly bigger protractor gives you more surface contact with the book, which helps too. But the real fix is the grip on the back.

Your angles are hard enough without fighting your tools.