HELP Quadratic formula NOOOOOOOO

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vladmire: Master | Next Rank: 500 Posts; Posts: 132; Joined: Tue Oct 07, 2008 4:59 pm; Thanked: 4 times

by vladmire » Tue Feb 03, 2009 5:53 pm

4x^2=9x

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Source: Beat The GMAT — Problem Solving | Tue Feb 03, 2009 5:53 pm

dimonya: Junior | Next Rank: 30 Posts; Posts: 28; Joined: Sun Feb 01, 2009 10:40 am; Location: Toronto

by dimonya » Tue Feb 03, 2009 9:16 pm

There are two methods for solving these

1. completing the square

2. Using formula (-b+-sqrt(b-4ac))/2a
i prefer to use 2

so first thing is ti het the equation on standard form (ax^2+bx+c=0)

in your case 4x^2-9x=0

where
a=4
b=9
c=0

answers follows

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Ian Stewart: GMAT Instructor; Posts: 2623; Joined: Mon Jun 02, 2008 3:17 amFollowed by: 355 members; Location: Montreal; Thanked: 1090 times; GMAT Score:780

by Ian Stewart » Wed Feb 04, 2009 5:16 am

dimonya wrote:There are two methods for solving these

1. completing the square

2. Using formula (-b+-sqrt(b-4ac))/2a
i prefer to use 2

Don't forget that it's b^2 - 4ac under the root, not b - 4ac. Fortunately we don't need to use the quadratic formula on the GMAT, as long as we know how to factor. So, if you need to solve for x in the question above:

4x^2 = 9x
4x^2 - 9x = 0
x(4x - 9) = 0

The product of two things is zero --> one of them must be zero:

x = 0
or
4x - 9 = 0 ---> x = 9/4

For online GMAT math tutoring, or to buy my higher-level Quant books and problem sets, contact me at ianstewartgmat at gmail.com

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