1) enter the value a
2) enter the value b
4) add the two numbers and store in f
3) verify the addition of two numbers with value in f

For the addition of two numbers  we have to test the 
following things...
1)Valid criteria
2)Invalid Criteria
3)Blank
Tc1:Valid data
1)Enter valid data for a &b check f value
Tc2:Blank
1)Enter value for a and keep b as blank and test the 
functionality
2)Enter value for b and check the functionality
3)Both are blanks check the functionality


1)a+ve value,B negative value(Vice versa)
2)A some value b is 0(vice versa)
3)Decimal values for both the fields
4)one decimal and one normal digit
5)Check for alphanumeric values
6)Enter space before,middle,end  entering the value 
andcheck the additon
7)special characters
.so many things are ther to consider the things ...

If anybody asks u to write testcases first ask them to give 
clear requirements so that depends upon that we can write 
the test cases..
here in this case we dont know the type of variables a&b
if we know the type of variables we can write as per that ..

please let me know that if  write anything wrong....