2014 Extension 2 BOS Trial Exam Discussion Thread (3 Viewers)

alficio

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Can someone help me with q10 multiple choice? I seriously have no clue
 

glittergal96

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Can someone help me with q10 multiple choice? I seriously have no clue
The original question had a typo, so you had to actually calculate it. (I did it via differentiation of the geometric series formula, a trick you see later in the exam incidentally).

With multiple choice though, you can almost always do these types of questions by doing little tests. If we set n=2 and w=-1, the sum we get is 1+2(-1-1)=-3. The only answer that gives -3 at n=2 is D.
 

Carrotsticks

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Can someone help me with q10 multiple choice? I seriously have no clue
The last option was supposed to be - 1-n.

This one was my fault. In the construction of the question, I summed two geometric series, one including w and another with w bar. I forgot though that I had to subtract 1 from both sides.
 
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glittergal96

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The last option was supposed to be - 1-n.

This one was my fault. In the construction of the question, I summed two geometric series, one including w and another with w bar. I forgot though that I had to subtract 1 from both sides.
Did you differentiate the geometric series formula or did you use some other trick for accounting for the linearly increasing coefficients? (The terms in the series being kz^{k-1}).
 

Carrotsticks

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To get the closed form initially I used the standard differentiation trick, I think in Trebla's solutions he also used the sum of the sum of gp method. There's a third way I know of using (1-x)S(x) trick that handles it nicely.
 

glittergal96

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To get the closed form initially I used the standard differentiation trick, I think in Trebla's solutions he also used the sum of the sum of gp method. There's a third way I know of using (1-x)S(x) trick that handles it nicely.
Ah yes, the differentiation and iterated sum methods were what came to mind, forgot about the (1-x)S(x) trick. I guess that reduces it to a GP, cool :).
 

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