√5, when the shadow of a bamboo pole with a length of 1 m is 2 meters long, Sina =1√ 5 can be obtained, and the points where the sun shines on the ball to form the shadow are all points, so the diameter (d) has a sinusoidal relationship with AB, that is, sinA=D/AB, and d = absina =/kloc-0.

Note: √5: the root sign is five; I suggest you draw a sketch. AB is the long side of a right triangle. I hope it helps you.