Question 841613: Find the sum of the first 8 terms if a1=18 and d=3
Answer by jim_thompson5910(35256)   (Show Source):
You can put this solution on YOUR website! First we need the nth term
an = a1 + d(n-1)
an = 18 + 3(n-1)
an = 18 + 3n-3
an = 3n + 15
Now plug in n = 8 into the nth term formula to find the 8th term a8
an = 3n + 15
a8 = 3(8) + 15
a8 = 39
Now use the values for n, a1, an to find S in the formula below
S = n*(a1 + an)/2
S = 8*(a1 + a8)/2 ... there are 8 terms, so n = 8
S = 8*(18 + 39)/2
S = 8*(57)/2
S = 456/2
S = 228
So the sum of the first 8 terms is 228
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