anonymous
  • anonymous
can someone please help me... I need to pass math. what is the sum of the series? A. -220 B. -110 C. -95 D. 110
Mathematics
katieb
  • katieb
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anonymous
  • anonymous
\[\sum_{n-1}^{20} (5-1)\]
anonymous
  • anonymous
that 1 in the parenthesis is supposed to be an n
anonymous
  • anonymous
I did the equation but when I add all the number up I get a weird number not one that i need.

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anonymous
  • anonymous
That's an arithmetic sequence because it's linear. You can use this formula |dw:1435279685117:dw|
anonymous
  • anonymous
\[\sum_{n=1}^{20} (5-n)\]?
anonymous
  • anonymous
I tried this but when I add the numbers I don't get the answer that i need... Could you help further on please.
anonymous
  • anonymous
you can do @peachpi way or you can break it in two \[\sum_{n=1}^{50}5-\sum_{n=1}^{20}n\]
anonymous
  • anonymous
oops \[\sum_{n=1}^{20}5-\sum_{n=1}^{20}n\]
anonymous
  • anonymous
I did 5 -1 -1 5 - 2 - 1 5 - 3 - 1 and so on
anonymous
  • anonymous
the first summation is \(20\times 5\) and for the second one it is \[\frac{20\times 21}{2}\]
anonymous
  • anonymous
use @peachpi formula, you do not want to add up twenty numbers
anonymous
  • anonymous
okay I will try
anonymous
  • anonymous
@peachpi can you write you formula out I dont really understand it
anonymous
  • anonymous
your formula is 5 -n. The sum goes from 1 to 20, so plug them in. What do you get when you sub in 1 for n? What do you get when you sub in 20 for n?
anonymous
  • anonymous
5 - 1 = 4 5 - 20 = -15
anonymous
  • anonymous
right...
anonymous
  • anonymous
|dw:1435280659076:dw|
anonymous
  • anonymous
-11
anonymous
  • anonymous
5*-11
anonymous
  • anonymous
-55
anonymous
  • anonymous
\[\frac{20}{2}\neq 5\]
anonymous
  • anonymous
10
anonymous
  • anonymous
anonymous
  • anonymous
so then 10 * -11
anonymous
  • anonymous
-110
anonymous
  • anonymous
yes
anonymous
  • anonymous
thank you so much.

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