If it weren't for the signs,
we would have the sequence 

5, 10, 15, 20, ...

So let's first get the nth term of that sequence.
The difference between any term and the preceding
term is 5. So the common difference d = 5.  The
nth term of an arithmetic sequence is given by:



 and 

Substituting,

 







Now we have to tack on a factor that
will cause the signs to alternate:

The alternating sign factors are 
and , depending on whether
the first term is negative or positive. Since 
the first term is -5, a negative number, we
use  for the alternating sign
factor.  So if we use (capital "A")  
for the nth term, we just multiply the nth term
for  (small "a") above by  
and get



Edwin