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### Course: Class 9 (Old)>Unit 5

Lesson 2: Parallel lines and a transversal

# Missing angles with a transversal

When a third line called a transversal crosses two parallel lines, we can find the measures of angles using properties like corresponding angles, vertical angles, and supplementary angles. If we know just one of the angle measurements, these properties help us find all the missing angle measurements. Created by Sal Khan.

## Want to join the conversation?

• can transversal line be perpendicular to parallel lines?
• Yes it can, it crosses through two lines at a certain point, so I would consider it transversal. It would create perpendicular lines.
• How do you remember the difference between supplementary and complementary?
• Remember:
Complementary angles add up to 90°
- example: 20° & 70°
(added together, they form a right angle)
-and-
Supplementary angles add up to 180°
- example: 60° & 120°
(added together, they form a straight line)

Two facts:
(1) 90° comes before 180° on the number line
(2) "C" comes before "S" in the alphabet

90° goes with "C" for complementary
so complementary angles add up to 90°

180° goes with "S" for supplementary
so supplementary angles add up to 180°

Hope this helps!
• What is a congruent angle?
• Congruent angles are, by definition, angles that have the same degree measure.
• How would I know if the lines are parallel? Therefore how would I know it actually is a transversal
• Bvanplane,
Either it has to be given that the lines are parallel,

Or you have to be given two angles that allow you to determine that like angles are equal,

Or you could be given an equation for the lines like
2x+3y=2 and
2x+3y = 8 which lines would have no solution, therefore they do not cross, therefore they are parallel.
• Why was "x" invented for math?!
(2 year later edit: Hi now that im in 6th i understand x and different kind of variables i was just too young to understand)
• "x" was invented as a variable to replace unknown numbers. So if I had 10 cookies in a jar and ate 3, (I know the answer is obvious) but say I didn't know how many are left. I could use x instead of a question mark. So 10 - 3 = x

Variables are also helpful because if I had two unknown numbers that were different in value, I could use x and y, instead of using the same ? for different values.

(x) is very helpful in life and I first learned about it in Pre-Algebra.
I hope I helped you.
"You only need to know one thing, you can learn anything"
• How did he find out what the pink angle was?
• The line where the transversal intercepts one of the parallel lines create 180 degrees due to the rule of supplementary angles. Supplementary is when two or more angles add up to 180 degrees. So Mr. Khan knew that the one measurement was 110 degrees. Using the rule of Supplementary angles, you know that the other side of the line must be 70 degrees, since 110 + 70 = 180. I hope you find this helpful!
• If complementary angles add up to 90 degrees and supplementary angles add up to 180, are there terms for angles that add up to 270 degrees and 260 degrees?
• Two angles that sum to a complete circle (1 turn, 360°) are called explementary angles or conjugate angles.
• how did he figure out that the angles are 70 degrees
• A straight line's degree angle is always 180 degrees. So if there is another line going through that 180 degree line, and you know the angle of that intersecting degree, you take that degree and minus it by 180.

So take that picture in the video for example, the angle he is working with is 110 degrees.
180 - 110 = 70