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netigen: Legendary Member; Posts: 631; Joined: Mon Feb 18, 2008 11:57 pmFollowed by: 3 members; Thanked: 29 times

by netigen » Wed Apr 23, 2008 12:25 am

a+b = 5
a^2+b^2 = 9
What is the value of X = ab/(sqrt13-sqrt5)

1. 9/5(sqrt13-sqrt5)
2. 5/9(sqrt13-sqrt5)
3. 16/(sqrt13-sqrt5)
4. sqrt13+sqrt5
5. sqrt5-sqrt13

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Source: Beat The GMAT — Problem Solving | Wed Apr 23, 2008 12:25 am

simplyjat: Master | Next Rank: 500 Posts; Posts: 423; Joined: Thu Dec 27, 2007 1:29 amFollowed by: 2 members; Location: Hyderabad, India; Thanked: 36 times; GMAT Score:770

by simplyjat » Wed Apr 23, 2008 1:19 am

a+b = 5
a^2+b^2 = 9
What is the value of X = ab/(sqrt13-sqrt5)

a^2+b^2 = (a+b)^2 - 2ab = 9
25 - 2ab = 9
2ab = 25 - 9 = 16
ab = 8

ab/(sqrt13-sqrt5)
= 8 / (sqrt13-sqrt5)
= 8 (sqrt13+sqrt5) / (13 - 5)
= (sqrt13+sqrt5)
...

simplyjat

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