3. 14×(20+ 10)2

=3. 14×900,

=2826 (square centimeter);

Transverse area of cylinder:

3. 14×20×20,

=3. 14×400,

= 1256 (square centimeter);

Use at least cardboard:

2826+ 1256=4082 (square centimeter);

A: At least 4082 square centimeters of cardboard should be used.

So the answer is: 4082.