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# help plz sigma notations a bit hard for me :/

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help plz sigma notations a bit hard for me :/

problem 1: Mar 21, 2019
edited by Melody  Mar 21, 2019

#1
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To do this question it is easier to work backwards from the qanswers.

$$\displaystyle\sum_{j=1}^7 \frac{1}{2}(j+1)\\ When\;\; j=1\quad \text{The first term is }\quad \frac{1}{2}(1+1)=1 \quad \text {that is good}\\ When \;\; j=2\quad \text{The second term is }\quad \frac{1}{2}(2+1)=\frac{3}{2}= \quad \text {that is no good}\\$$

So it is not the first one.

If you think about it, the bottom of the fractions are   $$2^0,2^1,2^2,....$$     and the top is just 1.   So the bottom is a power of 2.

Which out of the 4 seems likely?

Test that one, like I tested the first one,  and see if it works.

So what is the answer? Mar 21, 2019
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I'm still lost on how to do it.

Guest Mar 21, 2019
#3
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Not everything in life is instant!

Have you worked through how I looked at the first possibility and then dismissed it as wrong?

Melody  Mar 21, 2019
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So I believe it has to be either C or D.

Guest Mar 21, 2019
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Well have you tested either of them to see which one is right ?

Take the last one.

$$2^{j-1}$$

1) when j =1 what is the value?

2) when j=2 what is the value?

3) when j=3 waht is the value?

I want you to give me the answers to these questions.

Is this the right sequence?

Melody  Mar 21, 2019