Answer:
Length of a diagonal = 41 units
Explanation:
Given : A rectangle has a width of 9 units and a length of 40 units.
To Find : the length of a diagonal
Solution :
Since all angles of rectangle measures 90°.
So, to find length of diagonal use Pythagoras theorem i.e.

Refer attached figure
Since ΔADC is right angled triangle
So,

Where AC is the diagonal
AD = Length = 40 units
DC= Width = 9 units
Thus,




Hence, Length of a diagonal = 41 units