A line that crosses two parallel lines is called a transversal. For example, the line \overset{\longleftrightarrow}{EF} in the diagram below is a transversal.

A pair of parallel lines with a transversal form a set of 8 angles, shown below.

This set of eight angles can be grouped into two categories:

  • We have four pairs of vertical angles:
  • We also have four pairs of so-called corresponding angles:
    Corresponding pairs of angles are located in the same position as each other relative to the parallel lines and on the same side of the transversal. Pairs of corresponding angles are congruent.
FLAG

Consider the following pair of parallel lines \overset{\longleftrightarrow}{AB} \parallel \overset{\longleftrightarrow}{CD}, and the transversal \overset{\longleftrightarrow}{EF}.

What is the value of x?

First, note that \angle DIF and \angle BJI are corresponding angles. Therefore, they are congruent and thus have the same measure. This means we can write down the following equation:

\begin{align*} m\angle DIF &= m\angle BJI \\[5pt] 3x-20^\circ &= x + 10^\circ \end{align*}

To solve this equation, we first subtract x from both sides:

\begin{align*} 3x-20^\circ &= x + 10^\circ \\[5pt] 3x-20^\circ - x &= x + 10^\circ - x \\[5pt] 2x-20^\circ &= 10^\circ \\[5pt] \end{align*}

Then, we add 20^\circ to both sides:

\begin{align*} 2x-20^\circ &= 10^\circ \\[5pt] 2x-20^\circ+20^\circ &= 10^\circ+20^\circ \\[5pt] 2x&= 30^\circ \\[5pt] \end{align*}

Finally, we divide both sides by 2{:}

\begin{align*} \require{cancel} 2x&= 30^\circ \\[5pt] \dfrac{2x}{2}&= \dfrac{30^\circ}{2} \\[5pt] \dfrac{\cancel{2}x}{\cancel{2}}&= 15^\circ \\[5pt] x&= 15^\circ \end{align*}

Therefore, x=15^\circ.

FLAG

Solve for x in the figure above given that \overset{\longleftrightarrow}{AB} \parallel \overset{\longleftrightarrow}{CD}.

EXPLANATION

Since the angles \angle{CIF} and \angle{AJI} are corresponding angles, they are congruent and therefore have the same measure:

\begin{align*} m\angle{CIF} &=m\angle{AJI} \\[5pt] 5x + 10^\circ &= 110^\circ \end{align*}

Now, we have an equation that we can solve for x\mathbin{:}

\begin{align*} \require{cancel} 5x + 10^\circ &= 110^\circ \\[5pt] 5x &= 100^\circ \\[5pt] \dfrac{5x}{5} &= \dfrac{100^\circ}{5} \\[5pt] \dfrac{\cancel{5}x}{\cancel{5}} &= 20^\circ \\[5pt] x &= 20^{\circ} \end{align*}

FLAG

Solve for $y$ in the figure above given that $\overset{\longleftrightarrow}{AB} \parallel \overset{\longleftrightarrow}{CD}.$

a
$110^\circ$
b
$100^\circ$
c
$105^\circ$
d
$90^\circ$
e
$85^\circ$

Solve for $x$ in the figure above given that $\overset{\longleftrightarrow}{AB} \parallel \overset{\longleftrightarrow}{CD}.$

a
$8^\circ$
b
$15^\circ$
c
$7^\circ$
d
$24^\circ$
e
$18^\circ$

Solve for x in the figure above given that \overset{\longleftrightarrow}{AB} \parallel \overset{\longleftrightarrow}{CD}.

EXPLANATION

Since the angles \angle{AJF} and \angle{CIF} are corresponding angles, they are congruent and therefore have the same measure:

\begin{align*} m\angle{AJF} &= m\angle{CIF} \\[5pt] 9x - 10^\circ &= 7x + 4^\circ \end{align*}

Now, we have an equation that we can solve for x\mathbin{:}

\begin{align*} \require{cancel} 9x - 10^\circ &= 7x + 4^\circ \\[5pt] 9x - 7x &= 4^\circ + 10^\circ\\[5pt] 2x &= 14^\circ \\[5pt] \dfrac{2x}{2} &= \dfrac{14^\circ}{2} \\[5pt] \dfrac{\cancel{2}x}{\cancel{2}} &= 7^{\circ} \\[5pt] x &= 7^{\circ} \end{align*}

FLAG

Solve for $x$ in the figure above given that $\overset{\longleftrightarrow}{AB} \parallel \overset{\longleftrightarrow}{CD}.$

a
$13^\circ$
b
$16^\circ$
c
$23^\circ$
d
$9^\circ$
e
$6^\circ$

Solve for $x$ in the figure above given that $\overset{\longleftrightarrow}{AB} \parallel \overset{\longleftrightarrow}{CD}.$

a
$22^{\circ}$
b
$16^{\circ}$
c
$15^{\circ}$
d
$23^{\circ}$
e
$20^{\circ}$

Given that r , s and t in the above diagram are parallel, find m\angle{1}.

EXPLANATION

Consider \angle 2 and \angle 3 in the diagram below.

Note that \angle 2 and the angle whose measure is 100^\circ are corresponding angles formed by the same transversal. Therefore,

m\angle 2 = 100^\circ.

Similarly, note that \angle 3 and the angle whose measure is 50^\circ are corresponding angles formed by the same transversal. Therefore,

m\angle 3 = 50^\circ.

Let's add these measures to our diagram:

The angles \angle 1 , \angle 2, and \angle 3 combine to give a straight angle. So, we get

\begin{align} m\angle 1 +m\angle 2+ m\angle 3 &= 180^\circ \\[5pt] m\angle{1} + 100^\circ + 50^\circ &= 180^\circ \\[5pt] m\angle{1} + 150^\circ &= 180^\circ \\[5pt] m\angle{1} &= 30^\circ. \end{align}

FLAG

Given that $r$, $s$ and $t$ in the above diagram are parallel, find $m\angle{w}.$

a
$60^\circ$
b
$56^\circ$
c
$59^\circ$
d
$50^\circ$
e
$69^\circ$

Given that $r$, $s$ and $t$ in the above diagram are parallel, find $m\angle{1}.$

a
$52^\circ$
b
$43^\circ$
c
$30^\circ$
d
$40^\circ$
e
$45^\circ$

Solve for x in the figure above given that s \parallel t.

EXPLANATION

Consider \angle 1 and \angle 2 in the diagram below.

Note that \angle 1 and the angle whose measure is x are corresponding angles formed by the same transversal. Therefore,

m\angle 1 = x.

Similarly, note that \angle 2 and the angle whose measure is 2x are corresponding angles formed by the same transversal. Therefore,

m\angle 2 = 2x.

Let's add these measures to our diagram.

The angles \angle 1 , \angle 2, and the third angle whose measure is 45^\circ , combine to give a straight angle. So, we get

\begin{align*} \require{cancel} m \angle 1 + m\angle 2 + 45^\circ &=180^{\circ}\\[5pt] x + 2x + 45^\circ &=180^{\circ}\\[5pt] 3x + 45^{\circ} &=180^{\circ}\\[5pt] 3x &=135^{\circ}\\[5pt] \dfrac{3x}{3} &=\dfrac{135^{\circ}}{3}\\[5pt] \dfrac{\cancel{3}x}{\cancel{3}} &= 45^{\circ}\\[5pt] x &= 45^{\circ}. \end{align*}

FLAG

Solve for $x$ in the figure above given that $s \parallel t.$

a
$4^{\circ}$
b
$11^{\circ}$
c
$7^{\circ}$
d
$8^{\circ}$
e
$5^{\circ}$

Solve for $x$ in the figure above given that $s \parallel t.$

a
$20^{\circ}$
b
$17^{\circ}$
c
$12^{\circ}$
d
$22^{\circ}$
e
$15^{\circ}$
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