Two different lines either intersect at a single point, or they have no intersection. If they have no intersection, we say they are parallel.
Parallel lines are marked with chevrons like this: $>$>, and if the number of chevrons on two lines match, they are parallel.
If a pair of lines are not marked as parallel, how can we tell if they are or not? Is the intersection point off the edge of the diagram, or does it not exist at all? To tell these cases apart we introduce a third line, called a transversal, that intersects both lines.
The transversal forms angles at the intersection points, and there are three useful ways to identify them in pairs. The first kind are called cointerior, the two angles formed between the original lines, and on the same side of the transversal:
The second kind of pair are called alternate angles, lying between the original lines on opposite sides of the transversal:
The final kind of pair are called corresponding angles, lying in the same relative position on each line:
We can tell whether lines are parallel using one of these criteria:
This lesson refers to lines, but all of these ideas also apply to rays and segments. Rays and segments are parallel if the lines through their defining points are parallel.
You can explore the three kinds of angle pairs formed on parallel lines in the applets below:
If we know two lines are parallel, we can use these criteria to find the values of other angles.
If $\angle APB=82^\circ$∠APB=82°, what is $\angle DQE$∠DQE?
Think: The line through $A$A and $C$C is marked as parallel to the line through $F$F and $D$D. The line through $B$B and $E$E is a transversal. This means the cointerior angles at $P$P and $Q$Q are supplementary, the alternate angles are equal, and the corresponding angles are equal.
Do: $\angle APB=\angle FQB$∠APB=∠FQB, since they are corresponding angles on parallel lines.
$\angle FQB=\angle DQE$∠FQB=∠DQE, since these are vertically opposite angles.
This means that $\angle APB=\angle DQE$∠APB=∠DQE, so $\angle DQE=82^\circ$∠DQE=82°.
Reflect: We could have found the angle many different ways, using some of the other criteria. Here is another path we could have taken:
$\angle APB=\angle CPE$∠APB=∠CPE, since these are vertically opposite angles.
$\angle CPE=\angle DQE$∠CPE=∠DQE, since they are corresponding angles on parallel lines.
This means that $\angle APB=\angle DQE$∠APB=∠DQE, so $\angle DQE=82^\circ$∠DQE=82°, as before.
Can you find any others?
Select the diagram showing a pair of parallel lines:
Consider this diagram and answer the questions that follow:
Which two of the following are true statements?
Is there a pair of parallel lines in the diagram?
Solve for the value of $x$x.
Enter each line of working as an equation.