GMAT PREP I XY PLANE

[pkw209]

Hey all,

Couldn't figure this one out. A brief explanation would be very much appreciated. Thanks!

49) In the xy plane, at what point does y = (x + a)(x + b) cross the x axis?

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

[pagalmes]

To solve the problem, you need to know the values of a and b. Thus, you will need to equations including a and b.

- a. "a+b = -1" is the first equation, but is not enough on its own to solve the problem.
- b. allows you to set a second equation:

y = (x + a)(x + b)
-6 = (0 + a)(0 + b)
-6 = a.b

So now you have a system, and can find the a and b value :

a+b = -1
a.b = -6

You can now answer the DS problem. You need both together to answer.

If you want to know the points, you will find that the values for (a, b) are : (2, -3) or (-3, 2)
Finaly

y = (x - 3)(x + 2) crosses the x axis means "y = 0"

You can deduce the two points:

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

[sreak1089]

Is the Answer E? I think so, because we are not able to provide a single point at which the the graph intersects the x-axis
or is the answer C?

[pagalmes]

The original question (GMATPrep practice test 1, question 19) is:

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

So answer is C.