The summation formula of arithmetic sequence is known in arithmetic sequence {an}, A5 = 6, a8 = 15, A14?

The summation formula of arithmetic sequence is known in arithmetic sequence {an}, A5 = 6, a8 = 15, A14?

A8-a5 = 3D = 9, d = 3, so A14 = A8 + 6D = 33

It is known that {an} is an arithmetic sequence. If a1 + A5 + A9 = π, then the value of COS (A2 + A8) is ()
A. −12B. −32C. 12D. 32

Because {an} is an arithmetic sequence, and a1 + A5 + A9 = π, according to the property of arithmetic sequence, there is A5 = π 3, so A2 + A8 = 2 π 3, so cos (A2 + A8) = - 12, so choose & nbsp; a

It is known that {an} is an arithmetic sequence, if a1 + A5 + A9 = 2 π, then cos (A2 + A8) =?

Very simple! A1 + A5 + A9 = 3A5 = 2 Wu. So: A5 = 2 Wu / 3