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XAT 2015 Quant & DI: Quadratic Equations

This question appeared as a part of the quantitative reasoning and data interpretation section of the XAT 2015. Question 8 of a total of 33 questions that appeared in this section in XAT 2015. This one is an easy aptitude / quant question that tests concepts about quadratic equations and the curves that these equations represent.

Question

Find the equation of the graph shown below.

parabola

  1. y = 3x - 4
  2. y = 2x2 - 40
  3. x = 2y2 - 40
  4. y = 2x2 + 3x - 19
  5. x = 2y2 + 3y - 19

  Correct Answer      Choice E. x = 2y2 + 3y - 19


Explanatory Answer

  • Given Data

    In the graph, x and y-axes have been swapped from the conventional ones.
    i.e,. horizontal axis has been named y-axis and the vertical axis has been named x-axis.

    Key Inference

    The curve is a parabola.
    A parabola will have an equation of the form y = ax2 + bx + c
    However, in our case, the x and y axes have been swapped.
    So, the equation has to be of the form x = ay2 + by + c

    Points of Interest

    To determine which of the equations fits the curve given, we are going to substitute values of x or y from the graph in the equations and check for a match. We need two to three points to determine that. Let us select points where at least one of the coordinates is 0. The points of interest therefore are:
    1. When y is around -4, x = 0
    2. When y is between 2 and 3, x = 0
    3. When y is 0, x is around -20

    What is the approach?

    Substitute x = 0 in the choices that match the form of equation stated in the key inference step and see which one matches the data.
    Cross check with another value when y = 0.
    The choice that matches both conditions is the answer.

  • Compute the equation

    The equation has to be of the form x = ay2 + by + c.
    A quick look at the choices tells us that we can eliminate choices A, B, and D.
    Choice A is a linear equation. So, it represents a straight line. Rule it out.
    Choices B and D are of the form y = ax2. Our axes are swapped. So, choice B can also be ruled out.

    Choice C: x = 2y2 – 40
    Substitute y = -4 in the equation.
    We get x = 2(-4)2 – 40 = 32 – 40 = -8.
    However, we know that when y = -4, x = 0.

    So, Choice C cannot be the answer.

    Choice E: x = 2y2 + 3y – 19
    When y = -4, x = 2(-4)2 +3(-4) – 19 = 32 – 31 = 1.
    So, when y is around -4, it will be 0.

    Let us validate it with one more data point.
    When y = 0, x = 2(0)2 + 3(0) – 19 = -19.
    The value of x is around -20 as indicated in the curve.

      The correct answer is Choice E.

Video Explanation

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