In the key of A, chords I, IV and V will come from the A major scale.
Chord V here is an E chord, which may also be extended to become E7. Here are common fingerings for both these chords.
In Blues it is common to use 7 ths for all the chords. This 12 bar Blues in the key of A uses a combination of major chords and 7th chords. Notice how the 7ths make this one sound more bluesy than the previous exercise using only major chords. The staccato technique is used once again, this time with a slightly different strumming pattern.