In solving problems, we often encounter "midpoint", so how do we use the particularity of midpoint position to solve problems? First of all, let's understand several theorems related to the midpoint:

① The midline, height and bisector of the top angle on the bottom of the isosceles triangle coincide;

② The median line on the hypotenuse of the right triangle is equal to half of the hypotenuse;

③ The midline of the triangle is parallel to the third side, which is equal to half of the third side; The midline of the trapezoid is parallel to the two bottom sides and equal to half of the sum of the two bottom sides;

④ A straight line passing through the midpoint of one side of a triangle and parallel to the other side will bisect the third side; A straight line passing through the midpoint of one waist of the trapezoid and parallel to the bottom will bisect the other waist.