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### Course: Geometry (all content)>Unit 2

Lesson 7: Angles between intersecting lines

# Missing angles (CA geometry)

46-50, deducing the measure of angles. Created by Sal Khan.

## Want to join the conversation?

• I need some help here. At sal makes a mistake adding 2 to 168. The question (47) states 2x, 6x, 4x-6, 2x-16 and 6x MINUS 2, not 6x PLUS 2. This means that the biggest angle is 6x, since 6x+2 does not exist.
This means that the numbers are:
2x
6x
4x-6
2x-16
6x-2
=
20x-24=(3*180)
20x-24=540
20x=564
x=28.2

6x=169.2

None of the answer possibilities are correct unless I made an error. Can someone double check?
• You read the problem wrong. That's a plus sign, but the vertical line is really light. There's nothing more to it. *smile*

I hope this helps!
• Why does the sum of the three angles in a triangle always add up to 180 degrees?
• I don't understand how this video relates to the rest of the videos in this unit. Can someone explain? I don't remember Sal previously going through all of these concepts.
• I am with you Kim. These videos are in sequence and then all of a sudden he is showing problems with sums of angles of shapes and none of this has been mentioned in previous videos up to this point.
• Has it been proved that the more sides a polygon has, the more triangles? I noticed that a triangle has one triangle inside, a square has two, a pentagon has three, a hexagon has four, and etcedara. Does this pattern continue indefinitely?
• Yes, and you can use it to calculate things like the sum of the interior angles of a polygon. Good for you figuring that out!
• At what does Sal mean when he says that each triangle has 180 degrees?
• All of the internal angles of a triangle will always add up to 180°.
• Just out of curiosity, what is the formula for the degrees in a figure?
• The actual formula (for polygons) is (n-2)*180. n is the number of sides, then you subtract 2 and multiply it by 180.
• For question 50, couldn't Sal have just used 360/number of sides the polygon has? They get the same answer so I'm confused on why he took the long way...
• I think the way you're doing it is OK, it's just that he's making this for people who haven't done this type of math, so they can learn it.
• Since two lines are parallel if they never intersect, would all line segments that don't intersect be parallel?
• Well, a line segment has a defined beginning and end while a line doesn't. So, if it's shown that there are two line segments that aren't intersecting we know they can never intersect, but that doesn't mean that they're parallel. For them to be parallel, they have to be heading in the exact same direction (which means they'd also have the same slope).