2, 3, 5, 7, 8, 10, 15, …… is not an A.P.

∵ a₂ – a₁ = – =

a₃ – a₂ = – =

a₄ – a₃ = – =

i.e., The difference between any successive terms is not same throughout the .

The given list does not form an A.P, since each term is not obtained by adding a fixed number to its term.

-1,-3,-5,-7,….. is an A.P.

a₂ – a₁ = – – (- ) = - + = -

a₃ – a₂ = – – (-) = - + = -

a₄ – a₃ = – – (- ) = - + = -

Every number is formed by adding a fixed number to its term,