Help me with this DS questions frens

Expert replies

luckylucky: Junior | Next Rank: 30 Posts; Posts: 17; Joined: Mon Dec 01, 2008 8:37 pm

by luckylucky » Tue Feb 24, 2009 1:46 am

In the x-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 - axix at (0 , -6)

As usual, OA after some discussion

Like this post

Source: Beat The GMAT — Data Sufficiency | Tue Feb 24, 2009 1:46 am

DanaJ: Site Admin; Posts: 2567; Joined: Thu Jan 01, 2009 10:05 amFollowed by: 550 members; Thanked: 712 times; GMAT Score:770

by DanaJ » Tue Feb 24, 2009 2:15 am

Intersecting the X-axis means that y = 0. This is accomplished when x is either -a or -b. So in order to solve this problem, you need to know a and b.
y will also be x^2 + (a +b)x + ab.
1. a + b = -1 is not enough to find a and b, so this one is insufficient
2. this tells us that point (0, -6) is on the graph or that -6 = ab. Again, this is not enough to solve for a and b, since you have two unknowns and only one equation.

But put the two together and you can solve the problem. Notice that -6 is -3*2, which will be a and b (since it is consistent with a + b = -1), so we have solved the problem.

I'd go with C.

EDIT: Thank you, missed that.

Last edited by DanaJ on Tue Feb 24, 2009 10:11 am, edited 1 time in total.

Like this post

mrsmarthi: Senior | Next Rank: 100 Posts; Posts: 45; Joined: Sun Nov 30, 2008 8:41 pm; Thanked: 4 times

by mrsmarthi » Tue Feb 24, 2009 9:55 am

DanaJ wrote:Intersecting the X-axis means that y = 0. This is accomplished when x is either -a or -b. So in order to solve this problem, you need to know a and b.
y will also be x^2 + (a +b)x + ab.
1. a + b = -1 is not enough to find a and b, so this one is insufficient
2. this tells us that point (0, -6) is on the graph or that 0 = 36 - 6(a + b) + ab. Again, this is not enough to solve for a and b, since you have two unknowns and only one equation.

But put the two together and you get that 0 = 36 + 6 + ab or that ab = - 42. Notice that -42 is -7*6, which will be a and b (since it is consistent with a + b = -1), so we have solved the problem.

I'd go with C.

I will agree with the approach. But solutioning(which obviously is not needed is little incorrect)

2. this tells us that point (0, -6) is on the graph or that x = 0 and y = -6 ==> solving for x and y, we get ab = -6. Again, this is not enough to solve for a and b, since you have two unknowns and only one equation.

But putting together we have,

a+b = -1 and ab = -6. Two distinct eqs and two unknowns. So we should be able to get solve for X intercepts.

I will go with C.

Like this post

luckylucky: Junior | Next Rank: 30 Posts; Posts: 17; Joined: Mon Dec 01, 2008 8:37 pm

by luckylucky » Tue Feb 24, 2009 6:09 pm

Thank u guys ...

The OA is C

Like this post

Post new topic Post Reply

• Page 1 of 1

GMAT Course Reviews

560 reviews

1168 reviews

431 reviews

359 reviews

147 reviews

5/5

5 Star (560 Reviews)

"Target Test Prep is the closest to the official version of the GMAT exam, about 99% accuracy in terms of the quality and quantity of information. The course has excellently created singular sets of focused lessons and tests for every possible topic that one could come across in the official GMAT exam."

"The TTP course maximizes the efficiency of the time you spend studying. It will take time and effort but I could almost guarantee that if you complete the course exactly as it is laid out you will get an amazing score. They also have a very responsive team willing to help with any questions you might have."

"TTP has two things that I think no other test prep company offers: A teaching approach that reinforces understanding and an attitude that will give you the mental preparedness needed to succeed on the test. TTP gives you a deep understanding of the concept you need to know while teaching you how to think."

Write a Review