1. Fold the chart along a straight line. If it can completely coincide with another figure, the two figures are said to be symmetrical about this line. This straight line is called the axis of symmetry. The overlapping points after folding are the corresponding points, which are called symmetrical points.

2. The point where the distance between the two ends of a line segment is equal is on the middle vertical line of the line segment.

If the two angles of a triangle are equal, then the opposite sides of the two angles are equal.

4. The base angle of an isosceles triangle can only be acute, not obtuse (or right), and the top angle can be obtuse (or right).

5. If the median line of one side of a triangle is perpendicular to this side (the diagonal line bisecting this side), then this triangle is an isosceles triangle.