Answer :
I would recommend using:
a2 + b2 = c2
a and b are the legs, and c is the hypotenuse( or diagonal)
9•9
40•40
81+1600=c2
1681=c2
c=41
So the diagonal is 41units
a2 + b2 = c2
a and b are the legs, and c is the hypotenuse( or diagonal)
9•9
40•40
81+1600=c2
1681=c2
c=41
So the diagonal is 41units