Whenever two parallel lines are cut by a transversal, any pair of adjacent angles (angles that have a common side) form a linear pair. In other words, they are supplementary, and the sum of their measures is 180^\circ.

In total, there are 8 pairs of adjacent angles. These are shown below.

FLAG

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

EXPLANATION

First, notice that \angle{FIC} and \angle{IJA} are corresponding angles. Therefore, their measures are equal. m\angle{FIC} = m\angle{IJA} = 6x + 13^\circ

Also, the angles \angle{IJA} and \angle{IJB} are supplementary. So, we have: \begin{align*} m\angle{IJA} + \angle{IJB} &= 180^\circ \\[5pt] 6x+13^\circ + 5x-31^\circ &= 180^\circ \\[5pt] 11x-18^\circ &= 180^\circ \\[5pt] 11x &= 198^\circ \\[5pt] x &= 18^\circ \end{align*}

FLAG

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

a
$40^\circ$
b
$27^\circ$
c
$35^\circ$
d
$30^\circ$
e
$33^\circ$

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

a
$3^\circ$
b
$9^\circ$
c
$14^\circ$
d
$7^\circ$
e
$11^\circ$

Consecutive interior angles are the pairs of angles that are consecutive (on the same side of the transversal) and interior to (inside of) the parallel lines. These pairs of angles are always supplementary.

Consecutive exterior angles are angles that are consecutive (on the same side of the transversal) and exterior to (outside of) the parallel lines. These pairs of angles are always supplementary.

FLAG

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

EXPLANATION

We notice that \angle{EID} and \angle{BJF} are consecutive interior angles. Therefore, they are supplementary, and the sum of their measures is 180^{\circ}.

So, we have:

\begin{align*} m\angle EID + m\angle BJF &= 180^{\circ} \\[5pt] (5x+36^\circ) + (2x+4^\circ) & = 180^{\circ}\\[5pt] 7x + 40^{\circ} & = 180^{\circ}\\[5pt] 7x & = 140^{\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
$90^\circ$
b
$80^\circ$
c
$65^\circ$
d
$60^\circ$
e
$70^\circ$

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

a
$39^\circ$
b
$35^\circ$
c
$37^\circ$
d
$36^\circ$
e
$33^\circ$

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

EXPLANATION

We notice that \angle{FID} and \angle{EJB} are consecutive exterior angles. Therefore, they are supplementary, and the sum of their measures is 180^\circ. Therefore, we have

\begin{align*} m\angle{FID} + \angle{EJB} &= 180^\circ \\[5pt] (x+25^\circ) + (4x+30^\circ) &= 180^\circ \\[5pt] 5x+55^\circ &= 180^\circ \\[5pt] 5x &= 125^{\circ}\\[5pt] x &= 25^{\circ}. \end{align*}

FLAG

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

a
$140^\circ$
b
$135^\circ$
c
$150^\circ$
d
$130^\circ$
e
$145^\circ$

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

a
$50^\circ$
b
$70^\circ$
c
$40^\circ$
d
$60^\circ$
e
$30^\circ$

Solve for x given that s \parallel t, and \begin{align*} m\angle{1} = x-11^{\circ}, \qquad m\angle{2} = 5x+2^{\circ}, \qquad m\angle{3} = 5x - 10^{\circ}. \end{align*}

EXPLANATION

Consider \angle 4 and \angle 5 in the diagram below.

First, notice that \angle{2} and \angle{4} are consecutive interior angles formed by the same transversal. Therefore, \begin{align*} m \angle{4} &= 180^{\circ} - m\angle{2} \\[5pt] &= 180^{\circ}-\left(5x+2^{\circ}\right) \\[5pt] &= 178^{\circ}-5x. \end{align*}

Now, notice that \angle{5} and \angle{1} are alternate interior angles formed by another transversal. Therefore, \begin{align*} m\angle{5} &=m \angle{1} \\[5pt] &= x-11^{\circ}. \end{align*}

Finally, angles \angle{3}, \angle{4}, and \angle{5} form a straight angle. So, we obtain:

\begin{align*} m\angle{3} + m\angle{4} + m\angle{5} &= 180^{\circ} \\[5pt] \left(5x - 10^{\circ}\right) + \left(178^{\circ}-5x\right) + \left(x-11^{\circ}\right) &= 180^{\circ} \\[5pt] x + 157^{\circ} &= 180^{\circ} \\[5pt] x &= 23^{\circ} \end{align*}

FLAG

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

a
$95^\circ$
b
$89^\circ$
c
$91^\circ$
d
$81^\circ$
e
$99^\circ$

Solve for $x$ given that $s \parallel t,$ and \begin{align*} m\angle{1} = 4x-10^{\circ}, \qquad m\angle{2} = x+19^{\circ}, \qquad m\angle{3} = 2x + 9^{\circ}. \end{align*}

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