What is the sum of the arithmetic series of odd numbers below?1 + 3 + 5 + ... + 25?

Given from question
a = 1
an = 25
b = a₂ - a = 3 - 1 = 2

Asked from question
Sn

Solution
The general formula for finding the sum of arithmetic series is
Sn = n/2 (a + an)

Because we don't know yet the value of n, we should find it first, with this,
an = a + d(n - 1)
25 = 1 + 2(n - 1)
25 = 1 + 2n - 2
25 = 2n - 1
2n = 24
n = 12

After finding the value of x, calculate the sum of series by the formula I mentioned above
Sn = n/2 (a + an)
S₁₂ = 12/2 (1 + 25)
S₁₂ = 6 (26)
S₁₂ = 156

The sum of the series is 156