SOLUTION: Given the first four terms and the number of terms in the series, what is the sum of the following arithmetic series? 20 + 27 + 34 + 41, n = 16

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Question 1084071: Given the first four terms and the number of terms in the series, what is the sum of the following arithmetic series?
20 + 27 + 34 + 41, n = 16
Found 2 solutions by Boreal, ikleyn:
Answer by Boreal(15235) About Me  (Show Source):
You can put this solution on YOUR website!
Sum is n(a1+an)/2
a1=20, and an=a1+15*7=125
therefore, sum is 16(20+125)/2=8*145=1160
check
20+27+34+41+48+55+62+69+76+83+90+97+104+111+118+125=1160

Answer by ikleyn(52794) About Me  (Show Source):
You can put this solution on YOUR website!
.
On arithmetic progressions, see introductory lessons
    - Arithmetic progressions
    - The proofs of the formulas for arithmetic progressions
    - Problems on arithmetic progressions
    - Word problems on arithmetic progressions
    - Solved problems on arithmetic progressions


Also,  you have this free of charge online textbook in ALGEBRA-II in this site
    - ALGEBRA-II - YOUR ONLINE TEXTBOOK.

The referred lessons are the part of this online textbook under the topic "Arithmetic progressions".