Question

ABCD is a square of each side 5 cm, then the length of its diagonal is​

Answer 1

A diagonal divides the square into two congruent right triangles, thus you can use the side of the square to find the length of the diagonal ( which would be the hypotenuse of the right angled triangle).
If the square has the side of s cm then ,
by using pythagorean theorem,
(diagonal)^2 = (side)^2 + (side)^2
which gives
diagonal = (side) X sqrt(2)
here
side = 5 cm therefore,
diagonal = 5 x sqrt(2)
= 5 x 0.707
= 7.07 cm