Crystal Report -how to hide formula field as per the condition

Question

Hello All,
How to Hide formula field in crystal report as per the condition
i have applied i have given two values

1) total cost
2)total amount
3)Discount Rate
if total cost > total amount
then in discount Rate =total amount-total cost

4) if total cost=total amount
then discount rate displaying in my report =0.00

5) but i don't want to display 0.00

how to hide discount rate field ......when it displaly 0.00