In Arithmetic Progression:
an = a1 + ( n - 1 ) d
where
a1 = the initial term
d = the common difference of successive members
an = the nth term
In this case:
a1 = 15
d = 13
a12 = a1 + 11 d
a12 = 15 + 11 ∙ 13
a12 = 15 + 143
a12 = 158
The sum of the first n terms of arithmetic progression:
Sn = n ( a1 + an ) / 2
S12 = 12 ∙ ( a1 + a12 ) / 2
S12 = 12 ∙ ( 15 + 158 ) / 2
S12 = 12 ∙ 173 / 2
S12 = 2076 / 2
S12 = 1038
What is the sum of the first twelve term of an A.P whose 1st term is 15 and the common difference is 13
2 answers
S12 = 12/2 (2*15 + 11*13) = 1038