SOLUTION: i really need help on these ones they got me really confused i would appreciate it if i could get some help answering these thank you find the sum of the arithmetic sequence ha

Question 35103: i really need help on these ones they got me really confused i would appreciate it if i could get some help answering these thank you
find the sum of the arithmetic sequence having the data given.
a1 = 5, an = 3, n = 20
a1 = 7, d = - 3, n = 20
a1 = 81, an = - 13, n = 20
a1 = 5, d = 7, n = 120
an = 5, d = -2, n = 12
In exercises 6-7, find the first term of the arithmetic sequence having the data given.
an = 48, Sn = 546, n = 26
Sn = 781, d = 3, n = 22

Found 2 solutions by stanbon, Prithwis:
Answer by stanbon(75887) (Show Source): You can put this solution on YOUR website!
a1 = 5, an = 3, n = 20
Your notation is a bit confusing.
a1 means the 1st term of the series = 3
You are summing the 1st 20 terms; correct?
What do you mean by an=3 ?
Cheers,
Stan H.
Answer by Prithwis(166) (Show Source): You can put this solution on YOUR website!
Sum S(n) = n/2(a1+an)
(where n = number of terms; a1 = first term; an = n-th term)
a1 = 5, an = 3, n = 20
=> S(20) = 10(8) = 80;
Try doing the rest as above;
If d is known instead of an, Sum S(n) = n[a1 + (n-1)d/2]
You can use one of the above formula to find the sum of all listed problems;
For last two problems, you will have to use: an = a1+(n-1)d too.
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