sequence

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phelps: Senior | Next Rank: 100 Posts; Posts: 76; Joined: Mon Mar 02, 2009 4:50 am

by phelps » Fri Jul 23, 2010 12:58 pm

In a certain sequence , the tern Xn is given by formula Xn= 2Xn-1/2 ( Xn-2) for all n greater than or equal to 2.If X0= 3 and X1= 2 what is the value of X3.

a. 2.5
b. 3.125
c. 4
d. 5
e. 6.75

Ans c

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Source: Beat The GMAT — Problem Solving | Fri Jul 23, 2010 12:58 pm

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Rahul@gurome: GMAT Instructor; Posts: 1179; Joined: Sun Apr 11, 2010 9:07 pmFollowed by: 88 members; Location: Milpitas, CA; Thanked: 447 times

by Rahul@gurome » Fri Jul 23, 2010 7:30 pm

I think the question is wrongly posted .
Xn should be Xn = 2X(n-1) - Â½*X(n-2).
So X2 = 2X1 - Â½ *X0 = 2*2 - Â½ * 3 = 4 - 3/2 = 5/2.
Or X3 = 2*5/2 - Â½ * 2 = 5 - 1 = 4.

The correct answer is hence C.

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