1. Parallel lines and vertical lines: Parallel lines refer to two lines that will never intersect on the same plane, and vertical lines refer to two lines that intersect at 90 degrees. Students may confuse these two concepts and think that vertical lines are parallel lines.
2. Sum of triangle internal angles: the sum of triangle internal angles is equal to 180 degrees. Students may forget this basic theorem, which leads to errors in the calculation of the sum of the internal angles of triangles.
3. Nature of isosceles triangle: An isosceles triangle refers to a triangle with two equal sides. Students may confuse the nature of isosceles triangle, for example, they cannot correctly judge whether the base angles of isosceles triangle are equal.
4. Nature of right triangle: A right triangle refers to a triangle with an angle of 90 degrees. Students may confuse the nature of a right triangle, for example, they cannot correctly judge whether the hypotenuse of a right triangle is equal to the sum of the other two sides.
5. Nature of similar triangles: similar triangles refers to triangles with the same shape but different sizes. Students may confuse the nature of similar triangles, for example, they cannot correctly judge whether the corresponding sides of two similar triangles are proportional.
6. Nature of the circle: A circle refers to a figure whose distances from all points to the center of the circle are equal. Students may confuse the nature of a circle, for example, they cannot correctly judge whether the diameter of a circle is equal to twice the radius.