Question

find the length of diagonal of a hexagonal prism if the length of each side is 2r ?

Explain briefly... as I didnt get how root 3 came

Answer 1

Did you draw the diagram?

The diagonal is the base of am isosceles triangle with two sides of length 2r.
This triangle can be divided into two smaller right triangles, with angles 30-60-90 degrees, and hypotenuse 2r.
As you surely know, the sides are in the ratio 1:√3:2, so in your hexagon, each triangle has sides
r, r√ 3, 2r
The diagonal is thus r√ 3 + r√ 3 = 2r√ 3

I told you this before, and you apparently did very little work in actually working it out.