Question 1077134
find the diagnol length of a squares:

a square have {{{all}}} {{{sides}}} equal length
we usually mark them with an {{{a}}}, and diagonal with {{{d}}} 

{{{drawing( 600, 600, -10, 10, -10, 10, 
green(line(1,4,1,0)),green(line(4,0,1,0)),
green(line(1,4,4,4)),green(line(4,4,4,0)),
locate(4.2,2,a),locate(2.2,4.2,a),locate(0.5,2,a),locate(2.2,-.4,a),

blue(line(1,0,4,4)),locate(2.5,2,d),
 graph( 600, 600, -10, 10, -10, 10, 0)) }}} 

as you can see, diagonal divides square into two right triangles and you use Pythagorean theorem to find the diagonal length of a square

{{{d^2=a^2+a^2=2a^2}}} if you are given {{{a}}}

or the side length 

{{{a^2=d^2/2}}} if you are given {{{d}}}