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Hi Geva,Geva@MasterGMAT wrote:Question is asking at what point does y equal zero. This happens in one of two cases: when x=-a, or when x=-b. thus, in order to answer the question, you need to know the values of a and b.
Stat. (1): one equation, two unknowns - cannot find a and b.
Stat. (2): what this tells you is that when x=0, y must equal -6. Plug these into the equation to get x=0 into the equation to get -6=(0+a)(0+b), or ab=-6. Again, one equation, two unknowns - cannot find a and b.
Combined: two equations, two unknowns - you can find a and b. Note that for this particular pair (the sum and product of a and b), you will find two solutions - either a=2 and b=-3, or vice Versa: a=-3, b=2 - but in both cases you know that the x intercepts are the reverse :3 and -2. Sufficient - answer is C.
In the xy plane, any curve of the form y = f(x) crosses the x-axis at those points where y becomes zero. In this case y = (x + a)(x + b) crosses the x axis at points x = -a and x = -b. Therefore we need to find the values of a and b.prachich1987 wrote: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)
The answer is not A, it's C.prachich1987 wrote:I understand that when a graph intersects at x axis, the y-coordinate would be 0.
Here we have two values for a =2,-3
& two values for b=2,-3
Hence there are two possible points
(2,0)
(-3,0)
How can the answer be A then?
Please advise where I am going wrong?
Anurag explained this point so 'Um...What he said"prachich1987 wrote:Hi Geva,Geva@MasterGMAT wrote:Question is asking at what point does y equal zero. This happens in one of two cases: when x=-a, or when x=-b. thus, in order to answer the question, you need to know the values of a and b.
Stat. (1): one equation, two unknowns - cannot find a and b.
Stat. (2): what this tells you is that when x=0, y must equal -6. Plug these into the equation to get x=0 into the equation to get -6=(0+a)(0+b), or ab=-6. Again, one equation, two unknowns - cannot find a and b.
Combined: two equations, two unknowns - you can find a and b. Note that for this particular pair (the sum and product of a and b), you will find two solutions - either a=2 and b=-3, or vice Versa: a=-3, b=2 - but in both cases you know that the x intercepts are the reverse :3 and -2. Sufficient - answer is C.
Thanks for the explanation
I understand that when a graph intersects at x axis, the y-coordinate would be 0.
Here we have two values for a =2,-3
& two values for b=2,-3
Hence there are two possible points
(2,0)
(-3,0)
How can the answer be A then?
Please advise where I am going wrong?
It was a type error to write "A"Anurag@Gurome wrote:The answer is not A, it's C.prachich1987 wrote:I understand that when a graph intersects at x axis, the y-coordinate would be 0.
Here we have two values for a =2,-3
& two values for b=2,-3
Hence there are two possible points
(2,0)
(-3,0)
How can the answer be A then?
Please advise where I am going wrong?
And you're correct. There are two possible values of each a and b. Thus either (a = -3, b = 2) or (a = 2, b = -3). But in both the cases the graph is going to intersect the x-axis at x = -3 and x = 2.
I know this GMATPREP question: I believe the original question DID ask for points (plural), rather than point (singular). In any case, the former's answer is C, for reasons explained; the latter answer's E, for the reason you provide.prachich1987 wrote:It was a type error to write "A"Anurag@Gurome wrote:The answer is not A, it's C.prachich1987 wrote:I understand that when a graph intersects at x axis, the y-coordinate would be 0.
Here we have two values for a =2,-3
& two values for b=2,-3
Hence there are two possible points
(2,0)
(-3,0)
How can the answer be A then?
Please advise where I am going wrong?
And you're correct. There are two possible values of each a and b. Thus either (a = -3, b = 2) or (a = 2, b = -3). But in both the cases the graph is going to intersect the x-axis at x = -3 and x = 2.
I actually mean to say "how can the answer be C"
The question is asking us "at what point does y = (x + a)(x + b) cross the x axis?"
But we have got two points.
Hence even after combining the two statements , we don't get a unique value as it is intersecting at both (2,0) & (-3,0)
Thanks GevaGeva@MasterGMAT wrote:I know this GMATPREP question: I believe the original question DID ask for points (plural), rather than point (singular). In any case, the former's answer is C, for reasons explained; the latter answer's E, for the reason you provide.prachich1987 wrote:It was a type error to write "A"Anurag@Gurome wrote:The answer is not A, it's C.prachich1987 wrote:I understand that when a graph intersects at x axis, the y-coordinate would be 0.
Here we have two values for a =2,-3
& two values for b=2,-3
Hence there are two possible points
(2,0)
(-3,0)
How can the answer be A then?
Please advise where I am going wrong?
And you're correct. There are two possible values of each a and b. Thus either (a = -3, b = 2) or (a = 2, b = -3). But in both the cases the graph is going to intersect the x-axis at x = -3 and x = 2.
I actually mean to say "how can the answer be C"
The question is asking us "at what point does y = (x + a)(x + b) cross the x axis?"
But we have got two points.
Hence even after combining the two statements , we don't get a unique value as it is intersecting at both (2,0) & (-3,0)
BTG should put in a "like" buttonankurmit wrote:y = (x + a)(x + b)
y = x^2 +x (a+b) +ab
At point line crosses X-axis, Y=0
hence x^2+x(a+b)+ ab =0
Stem 1: Gives value of a+b but not value of ab
hence not sufficient
Stem 2:. graph intersects y axis at (0, -6)
y = x^2 +x (a+b) +ab
put values in equation we get value of ab
hence not sufficient
Combining both we get value of a+b and ab both
hence C is answer
.Since the equation is a binomial, there will be two points that satisfy this equation.prachich1987 wrote:It was a type error to write "A"Anurag@Gurome wrote:
The answer is not A, it's C.
And you're correct. There are two possible values of each a and b. Thus either (a = -3, b = 2) or (a = 2, b = -3). But in both the cases the graph is going to intersect the x-axis at x = -3 and x = 2.
I actually mean to say "how can the answer be C"
The question is asking us "at what point does y = (x + a)(x + b) cross the x axis?"
But we have got two points.
Hence even after combining the two statements , we don't get a unique value as it is intersecting at both (2,0) & (-3,0)
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