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Help me solve this - GMAT CD

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by joefreddy » Sat Jan 12, 2008 10:44 pm
In the xy plane, at what point does the graph of y = (x+a)(x+b) intersects the x-axis?

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

Answer - Choice C.

Please elaborate your answer. Thank you.
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by simplyjat » Sun Jan 13, 2008 2:09 am
When a graph intercepts X axis, Y = 0. Thus the question is asking what is the value of X in (x+a)(x+b) = 0
(x+a)(x+b) => x^2 + (a+b)x + ab = 0
now we need to know (a+b) & ab...

1. (a+b) = -1 gives value on one unknown.
2. This option simply states that when X = 0 Y = -6
so (0+a)(0+b) = -6 => ab = -6

knowing the value of two unknowns we can solve the original equal for the value of X
(x+2)(x-3) = 0, so X = -2 or 3....

So we know that the graph intersects X axis at two points -2 and 3 thus both statement together are sufficient
simplyjat
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by cris » Sun Jan 13, 2008 8:53 am
I dont see how it could be C. I think is it E.

Because with both statements we get that if y=0: x can be either 2 or -3.

1. From statement 1: we get 0=x^2+x+ab

2-. from statement 2 we get: -6=ab

3. Both statements together: 0=x^2+x-6 ...... x=either 2 or -3

Correct me if I am wrong
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by preciousrain7 » Mon Jan 14, 2008 2:16 pm
cris wrote:I dont see how it could be C. I think is it E.

Because with both statements we get that if y=0: x can be either 2 or -3.

1. From statement 1: we get 0=x^2+x+ab

2-. from statement 2 we get: -6=ab

3. Both statements together: 0=x^2+x-6 ...... x=either 2 or -3

Correct me if I am wrong
You can have two X-axis points when the equation is a parabola as seen here b/c y=x^2
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by cris » Mon Jan 14, 2008 3:26 pm
Hi preciusrain7! thanks for your post. I knew that it is has two points because it is a parabola...what I menat is that the question asks "at what point (singular) does...", and we have 2 points...so we dont know which of the two select...

This is the way DS problems work no? if we are asked for 1 point and with the info given we can calculate more than 1 and we can not know which one they are asking us for...then is E.

Could any one clarify this for me please, thanks!
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by camitava » Mon Jan 14, 2008 8:29 pm
cris, I got ur point. But the Qs is asking -
at what point does the graph of y = (x+a)(x+b) intersects the x-axis?
. In this case, as it is a parabola, it is having two X-intercepts. So it is not giving any extra answer but a detailed answer u can say. Say - (X1,0) and (X2,0) - these are the two points where the graph intercepts the X-axis.
As a conclusion, we can draw that it dos not mean in every case where we are getting two answers does not mean that we are failing to get the correct answer (As it is here). Are you getting me, Cris?
Correct me If I am wrong


Regards,

Amitava
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