Quadrilaterals (parallelograms, trapezoids), the Pythagorean theorem, and the concept of area.
In Atanasyan's problems, a bad drawing usually means a wrong answer. Compare your sketch with the solution's diagram to see if you missed a right angle or a midpoint. Common "Stumbling Blocks" in Atanasyan
If you’re stuck, look at just the first 1-2 lines of the solution to see which theorem was used (e.g., "By the property of isosceles triangles..."). Then, close the manual and try to finish it yourself.
Geometry isn't about the final answer (like "x=5"); it’s about the proof . Use GDZ to check if your deductive steps (the "Because... Therefore..." chain) match the formal requirements.