Suppose we have a pair of parallel lines cut by a transversal.

Alternate interior angles are the pairs of angles on alternate (opposite) sides of the transversal and are interior to (inside of) the parallel lines. These pairs of angles always have equal measure.

Alternate external angles are the pairs of angles on alternate (opposite) sides of the transversal and are exterior to (outside of) the parallel lines. These pairs of angles always have equal measure.

FLAG

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

EXPLANATION

The parallel lines \overset{\longleftrightarrow}{AB} \parallel \overset{\longleftrightarrow}{CD} are cut by a transversal \overset{\longleftrightarrow}{EF}.

The angles \angle{JID} and \angle{IJA} are interior to the parallel lines and are on opposite sides of the transversal. So, they are alternate interior angles, and consequently, they have the same measure.

Setting the measures of \angle{JID} and \angle{IJA} equal to each other, we have an equation that we can solve for x\mathbin{:}

\begin{align*} \require{cancel} m\angle{JID} &= m\angle{IJA} \\[5pt] 2x + 5^\circ &= 125^\circ \\[5pt] 2x &= 120^\circ \\[5pt] \dfrac{2x}{2} &= \dfrac{120^\circ}{2} \\[5pt] \dfrac{\cancel{2}x}{\cancel{2}} &= 60^\circ \\[5pt] x &= 60^\circ \end{align*}

FLAG

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

a
$26^\circ$
b
$31^\circ$
c
$30^\circ$
d
$33^\circ$
e
$28^\circ$

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

a
$50^\circ $
b
$30^\circ $
c
$35^\circ $
d
$45^\circ $
e
$40^\circ $

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

EXPLANATION

The parallel lines \overset{\longleftrightarrow}{AB} \parallel \overset{\longleftrightarrow}{CD} are cut by a transversal \overset{\longleftrightarrow}{EF}.

The angles \angle{FIC} and \angle{EJB} are exterior to the parallel lines and are on opposite sides of the transversal. So, they are alternate exterior angles, and consequently, they have the same measure.

Setting the measures of \angle{FIC} and \angle{EJB} equal to each other, we have an equation that we can solve for x\mathbin{:}

\begin{align*} \require{cancel} m\angle{FIC} &= m\angle{EJB} \\[5pt] x+39^\circ &= 3x-23^\circ \\[5pt] 39^\circ &= 2x-23^\circ \\[5pt] 62^\circ &= 2x \\[5pt] 2x & = 62^\circ \\[5pt] \dfrac{2x}{2} &= \dfrac{62^\circ}{2} \\[5pt] \dfrac{\cancel{2}x}{\cancel{2}} &= 31^\circ \\[5pt] x &= 31^\circ \end{align*}

FLAG

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

a
$24^\circ$
b
$19^\circ$
c
$16^\circ$
d
$22^\circ$
e
$18^\circ$

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

a
$32^\circ$
b
$25^\circ$
c
$24^\circ$
d
$28^\circ$
e
$30^\circ$

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

EXPLANATION

Consider \angle 2 in the diagram below.

Notice that \angle 2 and the angle whose measure is 65^\circ are alternate exterior angles formed when the transversal v cuts the parallel lines r and s. Hence, m\angle 2 = 65^{\circ}.

Also, notice that the angle determined by \angle 1 and \angle 2 and the angle whose measure is 112^\circ are corresponding angles formed when the transversal u cuts the parallel lines s and t. Hence, m\angle 1 + m\angle 2 = 112^{\circ}.

Now, substituting m\angle 2 into the equation, we can find m\angle1 \mathbin{:} \begin{align*} m\angle 1 + m\angle 2 & = 112^{\circ} \\[5pt] m\angle 1 + 65^\circ & = 112^{\circ} \\[5pt] m\angle 1 & = 47^{\circ} \end{align*}

FLAG

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

a
$63^{\circ}$
b
$75^{\circ}$
c
$78^{\circ}$
d
$69^{\circ}$
e
$72^{\circ}$

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

a
$70^\circ$
b
$67^{\circ}$
c
$65^{\circ}$
d
$72^\circ$
e
$63^{\circ}$

Given that \overleftrightarrow{AB} \parallel \overleftrightarrow{CF}, what is the measure of \angle AFB?

EXPLANATION

Notice that \angle{BAF} and \angle{AFC} are alternate interior angles formed when the transversal \overleftrightarrow{AF} cuts the parallel lines \overleftrightarrow{AB} and \overleftrightarrow{CF}. Hence,

m\angle{AFC} = m\angle{BAF} = 70^{\circ}\,.

Also, notice that \angle{FBE} and \angle{BFC} are alternate interior angles formed when the transversal \overleftrightarrow{BF} cuts the parallel lines \overleftrightarrow{AB} and \overleftrightarrow{CF}. Hence,

m\angle{BFC} =m\angle{FBE} = 110^{\circ}\,.

Now, using the angle addition postulate, we can find \angle{AFB}\mathbin{:}

\begin{align*} m \angle{AFB} &=m \angle{BFC} - m \angle{AFC} \\[3pt] &= 110^{\circ}-70^{\circ} \\[3pt] &= 40^{\circ} \end{align*}

FLAG

Given that $\overleftrightarrow{HQ} \parallel \overleftrightarrow{GC},$ what is the measure of $\angle GVU?$

a
$46^{\circ}$
b
$72^{\circ}$
c
$62^{\circ}$
d
$76^{\circ}$
e
$52^{\circ}$

Given that $\overleftrightarrow{AB} \parallel \overleftrightarrow{CF},$ what is the measure of $\angle AFB?$

a
$75^{\circ}$
b
$65^{\circ}$
c
$68^{\circ}$
d
$66^{\circ}$
e
$71^{\circ}$
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