SOLUTION: Write the sum using summation notation, assuming the suggested pattern continues. -9 - 3 + 3 + 9 + ... + 81 Can you please explain so I get it, because i know how to write th

Algebra ->  Sequences-and-series -> SOLUTION: Write the sum using summation notation, assuming the suggested pattern continues. -9 - 3 + 3 + 9 + ... + 81 Can you please explain so I get it, because i know how to write th      Log On


   

Question 1092090: Write the sum using summation notation, assuming the suggested pattern continues.
-9 - 3 + 3 + 9 + ... + 81
Can you please explain so I get it, because i know how to write this in the simple way, i just get really confused when it comes to using sigma and everything..
Answer by Fombitz(32388) About Me  (Show Source):
You can put this solution on YOUR website!
Start with the starting value, -9.
Then to get each successive term find the amount added,
-3-%28-9%29=-3%2B9=6
So add 6 to each previous value to get the next.
Let's choose k as the index.
So,
-9%2B6%2A%28k-1%29 is the formula to get any value.
So for the first value, k=1,
-9%2B6%2A%281-1%29=-9
Second, k=2
-9%2B6%2A%282-1%29=-9%2B6=-3
And so on.
So then stick this into the summation notation.
You can simplify too,
-9%2B6%28k-1%29=-9%2B6k-6=6k-15
So,
S%5Bn%5D=sum%28%286k-15%29%2Ck=1%2Cn%29