If ana_nan is an AP, what is the value of an−k+an+ka_{n-k} + a_{n+k}an−k+an+k?
2an2a_n2an
ana_nan
an−k+an+ka_{n-k} + a_{n+k}an−k+an+k
a1+a2na_1 + a_{2n}a1+a2n