A square has side 4cm each . Find the diagonal of it .

Answer is 5.657 cm

5.657 cm I.e. 5.66

Hi!
Here is the solution,
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Lets consider the following square, whose each side is 4 cm.
=> AB = BC = CD = AD (=4 cm)
As, we know that the angle between the adjacent sides is of 90 
So, here lets take Triangle ABC, which is a right angled triangle.

Now, by Pythagorean Theorem,
(AC)² = (AB)² + (BC)²
=> (AC)² = (4)² + (4)² cm
=> (AC)² = 16 + 16 cm
=> (AC)² = 32 cm
=> AC = √32 cm 
=> AC =  (2 x 2 x 2 x 2 x 2)^½ cm
            = (2⁵)^½  cm
             = 2^(5/2) cm
             = 2 ^{(2 + 0.5)} cm
             = 2² x 2^(½) cm
             = 4(√2) cm 

As both the diagonals of a square are equal, so BD = AC,THUS,
AC  =  4(√2) cm 
BD  =  4(√2) cm ---- {ANS.} 
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Hope it helps!
Cheers!