Coordinate Plane

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queenisabella: Senior | Next Rank: 100 Posts; Posts: 47; Joined: Sun Feb 05, 2012 7:56 pmFollowed by: 3 members

by queenisabella » Sun Feb 12, 2012 10:30 am

Data Sufficiency

In the xy-plane, at what two points does the graph of y=(x+a)(x+b) intersect the x-axis?
(1) a+b=-1
(2) The graph intersects the y-axis at (0,-6).

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Source: Beat The GMAT — Data Sufficiency | Sun Feb 12, 2012 10:30 am

rijul007: Legendary Member; Posts: 588; Joined: Sun Oct 16, 2011 9:42 amFollowed by: 9 members; Location: New Delhi, India; Thanked: 130 times; GMAT Score:720

by rijul007 » Sun Feb 12, 2012 11:29 am

queenisabella wrote:Data Sufficiency

In the xy-plane, at what two points does the graph of y=(x+a)(x+b) intersect the x-axis?
(1) a+b=-1
(2) The graph intersects the y-axis at (0,-6).

y = x^2 +x(a+b) +ab

we need values of x for
x^2 + x(a+b)+ab = 0

(1) a+b=-1
x^2 -x +ab = 0

Insuff

(2) The graph intersects the y-axis at (0,-6).

-6 = (0+a)(0+b)
ab = -6

x^2 +x(a+b)-6=0

Insuff

Combining the two sts

x^2 -x -6 = 0
(x+2)(x-3)=0

(-2,0) and (3,0)

IMO : OPTION C

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