Mathematics problem of the third grade of Beijing Normal University Edition: The included angle between the height of isosceles triangle and the bottom of waist is equal to half of the top angle (proposition solving). As shown in the figure, draw an isosceles triangle at will, from the vertex a? Draw a vertical line, which just divides the vertex angle into two completely equal angles.

Draw a vertical line from point C to line AB.

In the picture, two triangles, ABD? And then what? BCE？ There are two angles equal. ? So, BCE? =? The angle is not good, so the angle between the height of the isosceles triangle and the waist bottom is equal to half of the vertex angle.