ALTERNATING SIGNS:

These will come up a lot!

We know how to generate the evens

2 + 4 + 6 + 8 + 10 + ...

But, what if the signs alternate?

2 - 4 + 6 - 8 + 10 - ...

Using one of these will fix it:

( -1 )^k or ( -1 )^( k + 1 ) or ( -1 )^( k - 1 )

Which of these you use depends on where you start your index and if the thing starts with a positive or a negative.  I know you don't believe me, so check it out:

the summation of 2k ( - 1 )^k as k goes goes from 1 to infinity = -2 + 4 - 6 + 8 - ... (the ( -1 ) is multiplied in)

the summation of 2k ( -1 )^( k + 1 ) as k goes from 1 to infinity = 2 - 4 + 6 - 8 + ...

the summation of 2k ( -1 )^( k - 1 ) as k goes from 1 to infinity = ___


TRY IT:

Write the sigma notation for

1- 3 + 5 - 7 + 9 - ...

Write the sigma notation for

-1 + 4 - 9 + 16 - 25 + ...