Question 15. Look at the picture. Each hexagon consists of six small equilateral triangles with AB as the base and AB as 2. The height is the width of two hexagons and consists of the heights of four small equilateral triangles. The side length of each small equilateral triangle is 1. According to Pythagorean theorem, the height is 2/2 of the root number 3, and the fourth is twice of the root number 3. Using the triangle area formula, it is the root number 3 of 2.