For simplicity, assume the triangles are side length of 1
Line AD is TWO of the equilateral triangles heights BC is just one of the side lengths = 1
Formula for height of equlateral triangle: sqrt 3/2 * side length = sqrt3/2 *1 = sqrt3/2
but you have two of these so 2 * sqrt3/2 = sqrt 3
Now ratio oa AD to BC is sqrt 3 / 1 = sqrt 3