The length and breadth of a rectangle are 21 cm and 20 cm respectively. What must be the length of its diagonal (in cm)?

Let the length of the rectangle be l=21 cm and the breadth be b=20 cm.
The formula for the diagonal (d) of a rectangle is:

d=
l
2
+b
2


​

Substituting the given values:

d=
21
2
+20
2


​

d=
441+400

​

d=
841

​

d=29 cm