Here, a1=a,a2=2a+1,a3=3a+2 and a4=4a+3
a2−a1=2a+1−a=a+1
a3−a2=3a+2−2a−1=a+1
a4−a3=4a+3−3a−2=a+1
a2−a1=a3−a2=a4−a3=a+1=Common difference
Since the difference between successive terms are same, it forms an AP.
The next three terms are,
a5=a+4d=a+4(a+1)=5a+4
a6=a+5d=a+5(a+1)=6a+5
a7=a+6d=a+6(a+1)=7a+6