GMAT Prep?? (Arithmetic Sequence) - Please HELP. - The Beat The GMAT Forum

GMAT Prep?? (Arithmetic Sequence) - Please HELP.

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dferm: Master | Next Rank: 500 Posts; Posts: 446; Joined: Thu Jul 26, 2007 1:07 pm; Thanked: 6 times

by dferm » Tue Aug 12, 2008 5:07 am

I am not understanding the logic behind these types of questions.... How can i
get a value.

Please Help....

But please give me explanation of how to arrive to the answer..

Thanks.

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Source: Beat The GMAT — Data Sufficiency | Tue Aug 12, 2008 5:07 am

santa_dem: Senior | Next Rank: 100 Posts; Posts: 48; Joined: Fri Aug 08, 2008 4:44 am; Thanked: 4 times; GMAT Score:750

by santa_dem » Tue Aug 12, 2008 5:34 am

statement 1 taken separately gives you the general term x(i) for i>1, so there is no way to deduct x(1).

statement 2 separately gives us clues only about x(5). No way to solve it either.

Taken together, we have:

x(i)=x(i-1)/2
and
x(5)=x(4)/x(4)+1

From these statements, we can say that x(4)+1=2, x(4)=1

Using the general term x(i)=x(i-1)/2, we can say that x(1)=2*x(2)=2*2*x(3)=2*2*2*x(4)=2*2*2*1=8

So using the statements separately, no solving.

Statements together, we have a solution.

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dferm: Master | Next Rank: 500 Posts; Posts: 446; Joined: Thu Jul 26, 2007 1:07 pm; Thanked: 6 times

by dferm » Tue Aug 12, 2008 5:43 am

I am still confused on how both of them are sufficient.... Can you please help clarifying a little more.....Break it down so that I can really understand.....it is registering in my brain...

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santa_dem: Senior | Next Rank: 100 Posts; Posts: 48; Joined: Fri Aug 08, 2008 4:44 am; Thanked: 4 times; GMAT Score:750

by santa_dem » Tue Aug 12, 2008 6:02 am

It is clear that neither alone is sufficient. So try to solve the problem using both statements.

the problem is to find x(1).

You have 2 statements:

x(i)=x(i-1)/2

x(5)=x(4)/x(4)+1.

Both statements are true.

So lets calculate x(5) based on the first statement:

x(5)=x(5-1)/2=x(4)/2 --------------------------------- (1)

but x(5) is also = x(4)/x(4)+1------------------------- (2)

Acording to (1) and (2), we have the following equation:

x(4)/2=x(4)/x(4)+1. Divide the equation by x(4).

1/2=1/x(4)+1

x(4)+1=2

x(4)=1.

So we know x(4).

From the first statement,
x4=x3/2 => x3=2*x4
x3=x2/2 => x2=2*x3=2*2*x4.
x2=x1/2 => x1=2*x2=2*2*2*x4.

x4=1, so x2=2*2*2*1=8.

So using both statements, we can solve this problem.

Hope i was clear enough, I'm not very good at explaining stuff.

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dferm: Master | Next Rank: 500 Posts; Posts: 446; Joined: Thu Jul 26, 2007 1:07 pm; Thanked: 6 times

by dferm » Tue Aug 12, 2008 6:33 am

I understand how to do the problem now....Had to kind of keep trying but finally understand your logic.....

Thanks....your the best....

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