find the length of the diagonal of a square of side 8meter correct to two places of decimals

Consider a square ABCD whose sides AB = BC = CD = AD = 8 m

Now, in right triangle ABCD, by pythagoras theorem,

AC² = AB² + BC²

⇒ AC² = 8² + 8² 

⇒ AC² = 128

⇒ AC = √128 = 8√2 cm

⇒ AC =  8 × 1.414 cm = 11.31 cm