If ana_nan is an AP, which of the following is always true?
an+am=an+ma_n + a_m = a_{n+m}an+am=an+m
an−an−1=a2−a1a_n - a_{n-1} = a_2 - a_1an−an−1=a2−a1
an2a_n^2an2 is an AP
a2n=2ana_{2n} = 2a_na2n=2an