Help on Precalculus

Precalculus formula:

precalculus sequences formulas and series are given as follows,
Let, a₁ be the first term in an arithmetic sequence and the difference between any two successive terms be "d". Then the n^th term of that arithmetic sequence is,

a_n = a₁ + (n - 1)d

The sum of n
initial and final terms in an arithmetic sequence will be a₁ and a_n respectively is,

S_n = (a₁ + a_n)