What is the sum of the first 18 terms of the arithmetic series 11+15+19+23+...?

1 Answer
Aug 21, 2017

810

Explanation:

The required formula is color(red)(S_n = n/2[2a +(n-1)d])

Where,

S_n is the sum of n terms
n= the number of terms, " "n=18
a = the first term, color(white)(xxxxxxx)a = 11
d= the common difference, " "d= 4

Substituting these values gives:

S_18 = 18/2[2(11)+(18-1)(4)]

= 9[22+17xx4]

=9[22+68]

=9[90]

=810