The first three terms can be calculated by substituting n=1,2,3
Let them be denoted by a,b,c respectively.
a=2
b=5
c=10
a+c=12
2b=10
If
a,b,c actually were in an A.P., then a+c would be equal to
2b which is not the case here. Hence, the given sequence in not an
A.P.