ACT Math : How to find out if lines are parallel

Study concepts, example questions & explanations for ACT Math

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Example Questions

Example Question #191 : Geometry

Which of the following lines is parallel to:

\(\displaystyle 4y-12x=2\)

 

Possible Answers:

\(\displaystyle x-\frac{1}{3}y=7\)

\(\displaystyle x+4y=10\)

\(\displaystyle x-3y=9\)

\(\displaystyle y=\frac{1}{3}x+2\)

Correct answer:

\(\displaystyle x-\frac{1}{3}y=7\)

Explanation:

First write the equation in slope intercept form. Add \(\displaystyle 12x\) to both sides to get \(\displaystyle 4y = 12x + 2\). Now divide both sides by \(\displaystyle 4\) to get \(\displaystyle y = 3x + 0.5\). The slope of this line is \(\displaystyle 3\), so any line that also has a slope of \(\displaystyle 3\) would be parallel to it. The correct answer is  \(\displaystyle x - \frac{1}{3}y = 7\).

Example Question #1 : How To Find Out If Lines Are Parallel

Which pair of linear equations represent parallel lines?

Possible Answers:

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

y=2x+5\(\displaystyle y=2x+5\)

y=-x+4\(\displaystyle y=-x+4\)

y=x+6\(\displaystyle y=x+6\)

y=x-5\(\displaystyle y=x-5\)

y=3x+5\(\displaystyle y=3x+5\)

y=x+2\(\displaystyle y=x+2\)

y=-x+2\(\displaystyle y=-x+2\)

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

y=x+4\(\displaystyle y=x+4\)

Correct answer:

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

y=2x+5\(\displaystyle y=2x+5\)

Explanation:

Parallel lines will always have equal slopes. The slope can be found quickly by observing the equation in slope-intercept form and seeing which number falls in the "m\(\displaystyle m\)" spot in the linear equation (y=mx+b)\(\displaystyle (y=mx+b)\)

We are looking for an answer choice in which both equations have the same m\(\displaystyle m\) value. Both lines in the correct answer have a slope of 2, therefore they are parallel.

Example Question #192 : Geometry

Which of the following equations represents a line that is parallel to the line represented by the equation \(\displaystyle 10x-4y=26\)?

Possible Answers:

\(\displaystyle y=-\frac{5}{2}x+1\)

\(\displaystyle y=\frac{2}{5}x+2\)

\(\displaystyle y=-\frac{2}{5}x+3\)

\(\displaystyle y=5x-1\)

\(\displaystyle y=\frac{5}{2}x+1\)

Correct answer:

\(\displaystyle y=\frac{5}{2}x+1\)

Explanation:

Lines are parallel when their slopes are the same.

First, we need to place the given equation in the slope-intercept form.

\(\displaystyle -4y=-10+26\)

\(\displaystyle y=\frac{10}{4}x-\frac{26}{4}\)

\(\displaystyle y=\frac{5}{2}x-\frac{13}{2}\)

Because the given line has the slope of \(\displaystyle \frac{5}{2}\), the line parallel to it must also have the same slope.

Example Question #46 : Lines

Line \(\displaystyle p\) passes through the points \(\displaystyle (4, 2)\) and \(\displaystyle (-2, -2)\). Line \(\displaystyle Q\) passes through the point \(\displaystyle (2,3)\) and has a \(\displaystyle \textup{y-intercept}\) of \(\displaystyle b=0\). Are the two lines parallel? If so, what is their slope? If not, what are their slopes?

Possible Answers:

Yes, the lines are parallel with a slope of \(\displaystyle m=\frac{2}{3}\).

No, the lines are not parallel. Line \(\displaystyle P\) has a slope of \(\displaystyle m=\frac{2}{3}\) and line \(\displaystyle Q\) has a slope of \(\displaystyle m=2\).

Yes, the lines are parallel with a slope of \(\displaystyle m=2\).

No, the lines are not parallel. Line \(\displaystyle P\) has a slope of \(\displaystyle m=2\) and line \(\displaystyle Q\) has slope \(\displaystyle m=\frac{2}{3}\).

Correct answer:

Yes, the lines are parallel with a slope of \(\displaystyle m=\frac{2}{3}\).

Explanation:

Finding slope for these two lines is as easy as applying the slope formula to the points each line contains. We know that line \(\displaystyle P\) contains points \(\displaystyle (4, 2)\) and \(\displaystyle (-2, -2)\), so we can apply our slope formula directly (pay attention to negative signs!)

\(\displaystyle m=\frac{y_2-y_1}{x_2-x_1} = \frac{2+2}{4+2} = \frac{4}{6} = \frac{2}{3}\).

Line \(\displaystyle Q\) contains point \(\displaystyle (2,3)\) and, since the y-intercept is always on the vertical axis, \(\displaystyle (0,0)\). Thus:

\(\displaystyle m=\frac{y_2-y_1}{x_2-x_1} = \frac{2-0}{3-0} = \frac{2}{3}\)

The two lines have the same slope, \(\displaystyle m=\frac{2}{3}\), and are thus identical.

Example Question #47 : Lines

Line \(\displaystyle A\) is described by the equation \(\displaystyle y=-3x+7\). Line \(\displaystyle B\) passes through the points \(\displaystyle (0,3)\) and \(\displaystyle (-3,0)\). Are the two lines parallel? If so, what is their slope? If not, what are their slopes?

Possible Answers:

No, the lines are not parallel. Line \(\displaystyle A\) has slope \(\displaystyle m=-3\) and line \(\displaystyle B\) has slope \(\displaystyle m=1\).

Yes, the lines are parallel, and both lines have slope \(\displaystyle m=-3\).

Yes, the lines are parallel, and both lines have slope \(\displaystyle m=1\).

No, the lines are not parallel. Line \(\displaystyle A\) has slope \(\displaystyle m=1\) and line \(\displaystyle B\) has slope \(\displaystyle m=-3\).

Correct answer:

No, the lines are not parallel. Line \(\displaystyle A\) has slope \(\displaystyle m=-3\) and line \(\displaystyle B\) has slope \(\displaystyle m=1\).

Explanation:

We are told at the beginning of this problem that line \(\displaystyle A\) is described by  \(\displaystyle y=-3x+7\). Since \(\displaystyle y=mx+b\) is our slope-intecept form, we can see that \(\displaystyle m=-3\) for this line. Since parallel lines have equal slopes, we must determine if line \(\displaystyle B\) has a slope of \(\displaystyle -3\).

 Since we know that \(\displaystyle B\) passes through points \(\displaystyle (0,3)\) and \(\displaystyle (-3,0)\), we can apply our slope formula:

 \(\displaystyle m=\frac{y_2-y_1}{x_2-x_1} = \frac{3-0}{0-(-3)} = \frac{3}{3} = 1\)

Thus, the slope of line \(\displaystyle B\) is 1. As the two lines do not have equal slopes, the lines are not parallel.

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