Question

Use summation notation.. pleas help?

Answer 1

B: `Sigma_(n=1)^12(-1)(-2)^n = -1, +2, -4, +8, -16 ... -2048`

Explanation:

If I understand you right, you want to know which of A, B, C, and D would give the seriues in question.

First we note that
`-1 =-2^0`
Therefore A and D can be eliminated, since the first term is `2^1`, not `2^0`.

We can also eliminate C, since `2^(n-1)` will always be positive for all values of n.

That leaves B, since (-2)^(n-1) will alternate between positive values for (n-1) being even (n-1=0, n-1=2 etc.) and negative for n-1 being odd (n-1=1, n-1=3 etc)

Answer 2

B

Explanation:

If you put n=1 into each answer:
A gives `-1(-2)=2`
B gives `-1(-2)^0=-1 xx 1=-1`
C gives`-1(2)^0=-1 xx 1=-1`
D gives `-1(2)=-2`

So you can cancel out A and D

If you put n=2 into
B gives `-1(-2)=2`
C gives `-1(2)=-2`

You can see B gives the first two numbers in the sequence