Members of of an arithmetical progression and arithmetic average
$$a_{n} = \frac{a_{M1}+a_{P1}}{2}$$
a_n - n-th member
a_M1 - (n-1)-th member
a_P1 - (n+1)-th member
a_M1 - (n-1)-th member
a_P1 - (n+1)-th member
Find
It is known that:
= 