Write the sum using summation notation, assuming the suggested pattern continues.

5 - 15 + 45 - 135 + ...

Question

Write the sum using summation notation, assuming the suggested pattern continues.

5 - 15 + 45 - 135 + ...

anonymous · Answer

first factor out 5 then it will be much easier

ankit042 · Answer

This is a geometric series with r = -3,a=5
Do you knoe the formula to calculate the sum of Geometric series?

anonymous · Answer

no lol xD what is it?

anonymous · Answer

5(1-3+9-27...) these are all powers of 3

anonymous · Answer

1 is zeroth power
3 is first power 
9 is secend power
and so on

ankit042 · Answer

You can check this video to learn more about Geometric series http://www.youtube.com/watch?v=pXo0bG4iAyg

anonymous · Answer

$$\sum_{oo}^{n=0}5*3^n ?$$

anonymous · Answer

oops switch the n=0 and oo

anonymous · Answer

yes but there is a tiny problem alternating sign

anonymous · Answer

if you will expend your summation you will get
5(1+3+9+27...)

anonymous · Answer

you need odd powers to have -1 in front

anonymous · Answer

$$\sum_{n=0}^{\infty}5*\left( -3 \right)^{n}or \sum_{n=0}^{\infty}5*\left( -1 \right)^{n}3^{n}$$

anonymous · Answer

yes well done

anonymous · Answer

OHH awesome thank you :)

anonymous · Answer

@surjithayer could ya give @le0n a medal too!