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Knewton Challenge Discussion – GMAT Data Sufficiency (Graph Intersect)

Posted on March 3, 2011 by

22

This is a tricky Data Sufficiency question that over 50% of students miss. Can you tackle it?

Try it out, then leave your answer in the comments. When you’re ready to see how you did, check out the video explanation from our GMAT teacher, Jess.

If f(x) = ax² + bx + c, and a does not equal 0, at what point does the graph of the function f(x) intersect the y-axis?

  1. The graph of f(x) intersects the x-axis exactly twice, at (-6, 0) and (-2, 0)
  2. a = 2

(A) Statement (1) ALONE is sufficient, but statement (2) alone is not sufficient.
(B) Statement (2) ALONE is sufficient, but statement (1) alone is not sufficient.
(C) BOTH statements TOGETHER are sufficient, but NEITHER statement ALONE is sufficient.
(D) EACH statement ALONE is sufficient.
(E) Statements (1) and (2) TOGETHER are NOT sufficient.

Posted in Challenge Discussions, GMAT, Quant Guide | 22 comments



  • Nisrine

    The answer is c

    • Chandra

      If I rephrase the question, it asks for what is C? because when graph intersects Y axis, x is 0 then f(x)=c = y intersect.
      1. rephrase: y intersect is 0 because at those two points graph is intersected X axis. so ax² + bx + c=0. since -6 and -2 are x intersects, (x+6)(x+2)=ax² + bx + c. if you solve this equation. c=12. graph intersects y at (0,12) — sufficient
      2. since we cannot get c value by knowing a vaule, it is isufficient

      • Ali

        your assumption that (x+6)(x+2)=ax² + bx + c is flawed because you could consider (2x+12)(2x+4)=ax² + bx + c which results in c=48 you can not get unique answer then 1 is not sufficient.

  • Nisrine

    The answer is c

    • Chandra

      If I rephrase the question, it asks for what is C? because when graph intersects Y axis, x is 0 then f(x)=c = y intersect.
      1. rephrase: y intersect is 0 because at those two points graph is intersected X axis. so ax² + bx + c=0. since -6 and -2 are x intersects, (x+6)(x+2)=ax² + bx + c. if you solve this equation. c=12. graph intersects y at (0,12) — sufficient
      2. since we cannot get c value by knowing a vaule, it is isufficient

      • Ali

        your assumption that (x+6)(x+2)=ax² + bx + c is flawed because you could consider (2x+12)(2x+4)=ax² + bx + c which results in c=48 you can not get unique answer then 1 is not sufficient.

  • Ira_bedi

    option C?

  • Ira_bedi

    option C?

  • Ali

    the Answer is option c since
    if we put (-6,0)and (-2,0) in fx ,we conclude the followin equations:
    36a-6b+c=0,4a-2b+c=0 then if we have the value of a we can caculate C as the intersection of fx and Y axis

  • Ali

    the Answer is option c since
    if we put (-6,0)and (-2,0) in fx ,we conclude the followin equations:
    36a-6b+c=0,4a-2b+c=0 then if we have the value of a we can caculate C as the intersection of fx and Y axis

  • MK

    The answer is C as three unknown variables necessitate solving three available equations – the number of unknown variables should be proportional to the number of available equations.

  • MK

    The answer is C as three unknown variables necessitate solving three available equations – the number of unknown variables should be proportional to the number of available equations.

  • Fikoss1

    statement 1 an are not suffitient

  • Fikoss1

    statement 1 an are not suffitient

  • Deidrdel

    Does anybody else find this interesting and why? This is not part of everday living.

  • Deidrdel

    Does anybody else find this interesting and why? This is not part of everday living.

  • Prashant Verma

    a alone is fine. you can get the quad equation – (x+6)(x+2)=0

    • Prashant Verma

      c is correct. just realized my mistake.

  • Prashant Verma

    a alone is fine. you can get the quad equation – (x+6)(x+2)=0

    • Prashant Verma

      c is correct. just realized my mistake.

  • Avaneesh9

    c both statements together

  • Guest

    it’s c