CD=20cm, passing through AE⊥BC at point A, D, E and DF⊥BC at point F respectively.

AE=DF= 12cm，ef = ad = 10 cm。 (1 min)

In Rt△ABE,

BE=AB2？ AE2=9 (cm) (1)

Similarly, cf = 16 cm. (1) can be found in three situations:

(1) as shown in figure 1, BC=BE+EF+CF=35(cm)( 1).

(2) As shown in Figure 2, BC = ef-be+cf =17 (cm) (1).

(3) As shown in Figure 3, BC=BE+EF-CF=3(cm)( 1).

To sum up, the length of the other bottom of trapezoidal cardboard is 35cm or 17cm or 3 cm.