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

1 Answer
Aug 21, 2017

Answer:

#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#