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Hello, quantum, this is my attempt to explain why it's D:

Quote:

In the sequence of nonzero numbers t1, t2, t3, …, tn, …, tn+1 = tn / 2 for all positive integers n. What is the value of t5? (1) t3 = 1/4 (2) t1 - t5 = 15/16

Here we have geometric progression, i.e. series where t2=t1*q, t3=t2*q, …, tn+1=tn*q. In our case, q=0.5. Also note that tn+1=t1*q^n

So, basically, to answer this question, it is sufficient to know the value of any of the tn.

1) Explicitly gives us the value for t3, so it’s sufficient.

2) So, let’s see if we can obtain the value of t1 from this statement, using the formula tn+1=t1*q^n:

In the sequence of nonzero numbers t1, t2, t3, …, tn, …, tn+1 = tn / 2 for all positive integers n. What is the value of t5? (1) t3 = 1/4 (2) t1 - t5 = 15/16

see attached

Attachments

sequence.gif [ 6.83 KiB | Viewed 3998 times ]

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Factorials were someone's attempt to make math look exciting!!!

Good explanations but I got confused at how you equated tn+1=tn/2? I thought the it was the entire expression that equaled to tn/2? sorry but I am a bit confused. Thanks.

Good explanations but I got confused at how you equated tn+1=tn/2? I thought the it was the entire expression that equaled to tn/2? sorry but I am a bit confused. Thanks.

In the sequence of nonzero numbers t1, t2, t3, …, tn, …, tn+1 = tn / 2 for all positive integers n. What is the value of t5?

Given: t_{n+1}=\frac{t_n}{2}. So t_2=\frac{t_1}{2}, t_3=\frac{t_2}{2}=\frac{t_1}{4}, t_4=\frac{t_3}{2}=\frac{t_1}{8}, ...

Basically we have geometric progression with common ratio \frac{1}{2}: t_1, \frac{t_1}{2}, \frac{t_1}{4}, \frac{t_1}{8}, ... --> t_n=\frac{t_1}{2^{n-1}}.

Question: t_5=\frac{t_1}{2^4}=?

(1) t_3=\frac{1}{4} --> we can get t_1 --> we can get t_5. Sufficient. (2) t_1-t_5=2^4*t_5-t_5=\frac{15}{16} --> we can get t_5. Sufficient.

Answer: D.

Generally for arithmetic (or geometric) progression if you know:

- any particular two terms, - any particular term and common difference (common ratio), - any particular term and the formula for n_th term, - the sum of the sequence and either any term or common difference (common ratio),

then you will be able to calculate any missing value of given sequence.

Re: In the sequence of nonzero numbers t1, t2, t3, , tn, , tn+1 [#permalink]
15 Apr 2014, 01:43

1

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