Question 309015
Basic facts:
1. Area of a square = {{{Side ^ 2}}}
2. Diagonal of a square = {{{sqrt(2*Side^2)}}}

Let us take the length of each side of the square as x meters
Area of the square = 40 sq mt

Therefore {{{x^2}}} = 40

Therefore length of the diagonal = {{{sqrt(2*x^2)}}}
= {{{sqrt(2*40)}}}
= {{{sqrt(80)}}}
= {{{4*sqrt(5)}}} mts