Confusing coordinates

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Sandman: Senior | Next Rank: 100 Posts; Posts: 48; Joined: Wed Mar 05, 2008 10:21 am

by Sandman » Tue Sep 09, 2008 1:16 am

In the xy coordinate plane, at what two points does the line y=(x+a)(x+b) intersect the x-axis?

a. a+b=-1
b. the graph intersects the y-axis at (0,-6)

Whats the ans ?

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Source: Beat The GMAT — Data Sufficiency | Tue Sep 09, 2008 1:16 am

parallel_chase: Legendary Member; Posts: 1153; Joined: Wed Jun 20, 2007 6:21 amFollowed by: 2 members; Thanked: 146 times

Re: Confusing coordinates

by parallel_chase » Tue Sep 09, 2008 1:50 am

Sandman wrote:In the xy coordinate plane, at what two points does the line y=(x+a)(x+b) intersect the x-axis?

a. a+b=-1
b. the graph intersects the y-axis at (0,-6)

Whats the ans ?

y=(x+a)(x+b)

Statement I and II are clearly insufficient.

x^2+ax+bx+ab

a+b = -1
ab = -6 [from second statement]

x^2 - x -6 = 0

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

Therefore the two points are

(3,0) and (-2,0) because in both cases we know that y has to be 0.

Hope this helps.

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Sandman: Senior | Next Rank: 100 Posts; Posts: 48; Joined: Wed Mar 05, 2008 10:21 am

by Sandman » Tue Sep 09, 2008 2:32 am

Thanx for the reply...but how did u get x^2-x-6=0 after combining the two statements..shouldnt it be x^2-x-6=y ? Sorry but i dint get the last part where u calculated the two pts..Can u explain that plz.

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parallel_chase: Legendary Member; Posts: 1153; Joined: Wed Jun 20, 2007 6:21 amFollowed by: 2 members; Thanked: 146 times

by parallel_chase » Tue Sep 09, 2008 2:42 am

Sandman wrote:Thanx for the reply...but how did u get x^2-x-6=0 after combining the two statements..shouldnt it be x^2-x-6=y ? Sorry but i dint get the last part where u calculated the two pts..Can u explain that plz.

y intercept will always be 0 when a line intersects at x axis.

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Sandman: Senior | Next Rank: 100 Posts; Posts: 48; Joined: Wed Mar 05, 2008 10:21 am

by Sandman » Tue Sep 09, 2008 4:10 am

Thanx..I think i get it now.

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