How to do the fifth question on page P 108 of the first volume of ninth grade mathematics, who knows that it is the People's Education Edition?
Connect m? N two points, and then the vertex is high at O point. Because it is isosceles, the vertical line is equally divided (vertex and bottom). There are two congruent triangles (a right triangle with an angle of 30 degrees), and the hypotenuse is 12mm, so the vertical line is 6mm, and mn is equal to twice the number of 6 3, so the opening B is 12 3mm.