GRE Math : How to find the equation of a curve

Study concepts, example questions & explanations for GRE Math

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Example Questions

Example Question #2 : X And Y Intercept

What is the slope of the line whose equation is \dpi{100} \small 8x+12y=20\(\displaystyle \dpi{100} \small 8x+12y=20\)?

Possible Answers:

\dpi{100} \small -\frac{3}{2}\(\displaystyle \dpi{100} \small -\frac{3}{2}\)

\dpi{100} \small \frac{2}{3}\(\displaystyle \dpi{100} \small \frac{2}{3}\)

\dpi{100} \small \frac{3}{2}\(\displaystyle \dpi{100} \small \frac{3}{2}\)

\dpi{100} \small -\frac{2}{3}\(\displaystyle \dpi{100} \small -\frac{2}{3}\)

\dpi{100} \small 2\(\displaystyle \dpi{100} \small 2\)

Correct answer:

\dpi{100} \small -\frac{2}{3}\(\displaystyle \dpi{100} \small -\frac{2}{3}\)

Explanation:

Solve for \dpi{100} \small y\(\displaystyle \dpi{100} \small y\) so that the equation resembles the \dpi{100} \small y=mx+b\(\displaystyle \dpi{100} \small y=mx+b\) form. This equation becomes \dpi{100} \small -\frac{2}{3}x+\frac{5}{3}\(\displaystyle \dpi{100} \small -\frac{2}{3}x+\frac{5}{3}\). In this form, the \dpi{100} \small m\(\displaystyle \dpi{100} \small m\) is the slope, which is \dpi{100} \small -\frac{2}{3}\(\displaystyle \dpi{100} \small -\frac{2}{3}\).

Example Question #1 : X And Y Intercept

Which of the following equations has a \(\displaystyle y\)-intercept of \(\displaystyle 13\)?

Possible Answers:

\(\displaystyle 3y=4x^2-16\)

\(\displaystyle 4x^2=12y+12\)

\(\displaystyle y=(x-4)^2-3\)

\(\displaystyle 2x^2-16y=5\)

\(\displaystyle 22x-2y=1\)

Correct answer:

\(\displaystyle y=(x-4)^2-3\)

Explanation:

To find the \(\displaystyle y\)-intercept, you need to find the value of the equation where \(\displaystyle x=0\).  The easiest way to do this is to substitute in \(\displaystyle 0\) for your value of \(\displaystyle x\) and see where you get \(\displaystyle 13\) for \(\displaystyle y\).  If you do this for each of your equations proposed as potential answers, you find that \(\displaystyle y=(x-4)^2-3\) is the answer.

Substitute in \(\displaystyle 0\) for \(\displaystyle x\):

\(\displaystyle y=(0-4)^2-3=(-4)^2-3=16-3=13\)

Example Question #111 : Coordinate Geometry

If \(\displaystyle m\) is a line that has a \(\displaystyle y\)-intercept of \(\displaystyle 3\) and an \(\displaystyle x\)-intercept of \(\displaystyle 7\), which of the following is the equation of a line that is perpendicular to \(\displaystyle m\)?

Possible Answers:

\(\displaystyle y=\frac{(7x+15)}{3}\)

\(\displaystyle y=\frac{x+7}{3}\)

\(\displaystyle y=\frac{(3x+11)}{7}\)

\(\displaystyle y=\frac{(-3x-24)}{7}\)

\(\displaystyle y=\frac{(7-7x)}{3}\)

Correct answer:

\(\displaystyle y=\frac{(7x+15)}{3}\)

Explanation:

If \(\displaystyle m\) has a \(\displaystyle y\)-intercept of \(\displaystyle 3\), then it must pass through the point \(\displaystyle (0,3)\).

If its \(\displaystyle x\)-intercept is \(\displaystyle 7\), then it must through the point \(\displaystyle (7,0)\).

The slope of this line is \(\displaystyle \frac{0-3}{7-0}=-\frac{3}{7}\).

Therefore, any line perpendicular to this line must have a slope equal to the negative reciprocal, which is \(\displaystyle \frac{7}{3}\). Only \(\displaystyle y=\frac{(7x+15)}{3}\) has a slope of \(\displaystyle \frac{7}{3}\).

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