hello could you please help me with this math problem with full explanation which I am unable to solve? Thanks a lot.

Area of a square is side^2
Diagonal of a square is sqrt(2) x side
Using this rewrite the area formula as :
1/2 x diagonal^2
Area = 1/2 x 40^2 = 800
Area = 800 m^2
Answer = b) 800m^2