Verify that each of the following is an $AP$,and then write its next three terms.
$a+b, (a+1)+b, (a+1)+(b+1), \ldots$

(N/A) Given sequence: $a_1 = a+b, a_2 = a+1+b, a_3 = a+1+b+1$.
Calculate the common difference $d$:
$d_1 = a_2 - a_1 = (a+1+b) - (a+b) = 1$.
$d_2 = a_3 - a_2 = (a+1+b+1) - (a+1+b) = 1$.
Since $d_1 = d_2 = 1$,the common difference is constant,so the sequence is an $AP$.
The next three terms are:
$a_4 = a_3 + d = (a+1+b+1) + 1 = a+b+3 = (a+2)+(b+1)$.
$a_5 = a_4 + d = (a+b+3) + 1 = a+b+4 = (a+2)+(b+2)$.
$a_6 = a_5 + d = (a+b+4) + 1 = a+b+5 = (a+3)+(b+2)$.