SOLUTION: Find a1 if d = 4, a20 = 82, and S20 = 880. A.) 2 B.) 4 C.) 6 D.) 60 is this the right rule? an = a1+(n-1)d Sn = (n/2)(a1+an) If so can I see a demonstration on

Question 1041057: Find a1 if d = 4, a20 = 82, and S20 = 880.
A.) 2
B.) 4
C.) 6
D.) 60
is this the right rule?
an = a1+(n-1)d
Sn = (n/2)(a1+an)
If so can I see a demonstration on how you solve it?
Found 2 solutions by stanbon, ikleyn:
Answer by stanbon(75887)   (Show Source): You can put this solution on YOUR website!
Find a1 if d = 4, a20 = 82, and S20 = 880.
A.) 2
B.) 4
C.) 6
D.) 60
----------------------
an = a1+(n-1)d
Sn = (n/2)(a1+an)
----------
a(20) = a(1) + 19*4 = a(1) + 76 = 82
a(1) = 82-76 = 6
------------------
S(20) = (20/2)(a(1) + a(20)) = 10(a(1) + 82)) = 880
a(1) + 82 = 88
a(1) = 6
----------------------
Cheers,
Stan H.
==================
Answer by ikleyn(52781)   (Show Source): You can put this solution on YOUR website!
.
On arithmetic progressions see the lesson
    - Arithmetic progressions
in this site.

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