anonymous
  • anonymous
find sum of the 1st 20 terms in the arithmetic series 5,8,11,14
Mathematics
  • Stacey Warren - Expert brainly.com
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SOLVED
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katieb
  • katieb
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amistre64
  • amistre64
what do we add to get to each next term?
amistre64
  • amistre64
called a common difference
anonymous
  • anonymous
3

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amistre64
  • amistre64
3 is right :) so, Sn = n(first+last)/2 if i recall it right
amistre64
  • amistre64
to determine the 20th term is a20 = 5+3(19) = 62 62+5 = 67*20/2 = 670 ... if my math was good
anonymous
  • anonymous
Oh so it's 670? Thank you so so so much! wow thank you.
amistre64
  • amistre64
i hope so, id dbl chk with the wolf: sum (5+3(n-1)) from 1 to 20
amistre64
  • amistre64
http://www.wolframalpha.com/input/?i=sum+%285%2B3%28n-1%29%29+from+1+to+20
amistre64
  • amistre64
670 :)
anonymous
  • anonymous
Can you help me with one more? Find the 7th term in the geometric sequence 4,20,11
amistre64
  • amistre64
20/4 = our common ratio an = a1 * r^(n-1) a7 = 4 * (20/5)^6
amistre64
  • amistre64
your 11 is bad there, is that a typo?
anonymous
  • anonymous
sorry i meant 100
anonymous
  • anonymous
yes :/
amistre64
  • amistre64
whew!! lol, then yeah; 4*5^6 should do it
anonymous
  • anonymous
so a^7 = 4*5^6?
anonymous
  • anonymous
dd you get 250?
amistre64
  • amistre64
i think its bigger than that; 5^4 = 25^2 = 625 alone
amistre64
  • amistre64
625*20 is what id end up with
amistre64
  • amistre64
12500 perhaps? hard to do in the head :)
amistre64
  • amistre64
http://www.wolframalpha.com/input/?i=4*%285%5E6%29 almost had it :)
amistre64
  • amistre64
i left off the last *5 .... 12500*5=62500

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