Answer:
=18975
Step-by-step explanation:
This is an arithmetic sequence
an = a1 +d(n-1)
a1 =9
d = 12-9 = 3
an = 9 +3 (n-1)
The sum of an arithmetic sequence is given by
Sn = n/2 (a1+an)
So we need to determine what number in the sequence 336 is
an = 9 +3 (n-1)
336 = 9 +3(n-1)
Subtract 9 from each side
336-9 =9+-9 + 3(n-1)
327 = 3(n-1)
Divide each side by 3
327/3 =3/3(n-1)
109 = n-1
Add 1 to each side
109+1= n-1+1
110 = n
336 is the 110 term
The sum from 9 to 336
Sn = n/2 (a1+an)
S 110 = 110/2 (9+336)
= 55 (345)
=18975
