1, the midline theorem of a triangle: the midline of a triangle is parallel to the third side and equal to half of it.

2. Diagonal lines of inscribed quadrangles of a circle are complementary, and any external angle is equal to its internal angle.

3. If the opposite side of the 30 internal angle of a triangle is half of one side, then this triangle is a right triangle with this long side as the hypotenuse.

4. Radius of cone bottom surface r = n/360 L (L is the length of bus) (R is the radius of bottom surface).

5. A straight line and a circle have two common points, which are called intersections. This straight line is called the secant of the circle, AB intersects ⊙O, D.