Question 976865: find a8 when a1 = -6, d=2. use the formula for the general term ( the n^th term ) of an arithmetic sequence to find the indicated term of the sequence with the given first term , a1 and common difference , d.
a. 10
b. 8
c. -20
d. -22
find the sum of the first 20 terms of the arithmetic sequence 5, 13, 21, 29
a. 1700
b. 1620
c. 1627
d. 165
Answer by Cromlix(4381)   (Show Source):
You can put this solution on YOUR website! Formula = a(n)= a(1) + (n - 1) d
a(1) = -6 d = 2
a(8) = -6 + (8 - 1)2
a(8) = -6 + 14
a (8) = 8
Find the Sum
a(1) = 5: n = 20: d = 8
a(n) = a(1) + (n - 1)d
a(20) = 5 + (20 -1)8
a(20) = 5 + 152
a(20) = 157
Sum formula = S(n) = n( a(1) + a(n))/2
Now, let's find the sum:
s(20) = [20( 5 + 157)]/2
s(20) = 3240/2 = 1620
Both answers are 'b'
Hope this helps:-)
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