plz help

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plz help

by rmazzawi » Wed Sep 14, 2011 2:29 am
how do i solve such questions ???

For what values of 'k' will the pair of equations 3x + 4y = 12 and kx + 12y = 30 not have a unique solution?
12
9
3
7.5
2.5
The correct choice is (B) and the correct answer is 9.

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by cans » Wed Sep 14, 2011 2:40 am
generally 2 lines either intersect at a point, or they don't intersect at all (when they are parallel)
Thus: to not have a unique solution, lines should be parallel.
slope of 1st = -3/4 = slope of 2nd = -k/12
3/4 = k/12 or k=9
IMO B
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by GMATGuruNY » Wed Sep 14, 2011 2:50 am
rmazzawi wrote:how do i solve such questions ???

For what values of 'k' will the pair of equations 3x + 4y = 12 and kx + 12y = 30 not have a unique solution?
12
9
3
7.5
2.5
The correct choice is (B) and the correct answer is 9.
Scan the answers for a likely candidate.
If k=9, the second equation becomes 9x+12y=30, which is the same as 3x+4y=10.
This equation contradicts the first equation: 3x+4y=12.

The correct answer is B.
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