find the least common denominators for these two rational expressions 1/a 8/8a^2

Next time, please separate the expressions with a comma, semicolon or the word "and."

I need to assume that you meant "8/(8a^2)"  If that's the case, then what you have here is

 1                  8
---     and -----------
 a                8a^2

I would then cancel the 8s, obtaining:

 1                  1
---     and -----------
 a                a^2
                                                            a
and then multiply  1/a by (a/a):      -----------
                                                           a^2

to obtain 

  a                1
------- and -------
  a^2           a^2

There is only one LCD here, and it's "a^2."