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klaud: Junior | Next Rank: 30 Posts; Posts: 27; Joined: Sat Jan 28, 2012 8:22 am; Followed by:1 members

equations

by klaud » Mon Feb 13, 2012 6:07 am

Question 1

Solve for x:

3x+2y=-1
4x+3y=1

Question 2

Solve for x+y:

7x+4y=19
4x+7y=3

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Source: Beat The GMAT — Problem Solving | Mon Feb 13, 2012 6:07 am

LalaB: Master | Next Rank: 500 Posts; Posts: 425; Joined: Wed Dec 08, 2010 9:00 am; Thanked: 56 times; Followed by:7 members; GMAT Score:690

by LalaB » Mon Feb 13, 2012 6:25 am

3x+2y=-1
4x+3y=1

12x+8y=-4
12x+9y=3
y=7
x=-5
===============

7x+4y=19
4x+7y=3

28x+16y=76
28x+49y=21
33y=-55
y=-5/3
x=11/3

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Anurag@Gurome: GMAT Instructor; Posts: 3835; Joined: Fri Apr 02, 2010 10:00 pm; Location: Milpitas, CA; Thanked: 1854 times; Followed by:523 members; GMAT Score:770

by Anurag@Gurome » Mon Feb 13, 2012 8:03 pm

klaud wrote:Question 1

Solve for x:

3x+2y=-1
4x+3y=1

Solve it by elimination method:

Make the coefficients of y equal in both the equations. We can do so by multiplying equation (1) by 3, and multiplying equation (2) by 2,

3x + 2y = -1 } * 3 implies 9x + 6y = -3
4x + 3y = 1 } * 2 implies 8x + 6y = 2
Now subtract both the equations,
So, 9x - 8x = -3 - 2
x = -5

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Anurag@Gurome: GMAT Instructor; Posts: 3835; Joined: Fri Apr 02, 2010 10:00 pm; Location: Milpitas, CA; Thanked: 1854 times; Followed by:523 members; GMAT Score:770