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### Course: 7th grade (Illustrative Mathematics) > Unit 7

Lesson 4: Lesson 5: Using equations to solve for unknown angles# Equation practice with supplementary angles

Given the algebraic expressions that represent a pair of supplementary angles, Sal forms and solves an equation. Created by Sal Khan.

## Want to join the conversation?

- At0:09Sal says, "And I'll assume it's in degrees." What other angle measurements are there, if any at all?(13 votes)
- There are two other measures of a turn of a circle. There are radians, and there are gradians. 1 radian is equivalent to the ratio of the circle's circumference to its diameter, also known as 2*pi, and gradians is equal to the ratio of the circle's circumference to its diameter, divided by 400; therefore a gradian is equivalent to 1/400 of a turn in a circle, 9/10 of 1 degree, or pi/200 of a radian.(15 votes)

- Lines AB and CD intersect at M. If the m∠AMD = 2x + 20 and the m∠DMB = 3x + 60, find the measure of ∠AMC.(6 votes)
- I think I can figure your problem out. First, is your angle supplementary or complementary? If the angle is supplementary, the equation for x would be 2x+20+3x+60=180. If the angle is complementary, the equation would be 2x+20+3x+60=90. The answer for supplementary would be 60. The answer for complementary would be 24.

Hope this helped!(0 votes)

- hi sal, i want to tell you can you add missing numbers in equations(7 votes)
- How can we assume it forms a line based on how it looks? Shouldn't it be specified in the question?(3 votes)
- I agree. I took geometry last year and the one thing that we had to "assume" sometimes was that a line was not two rays. Is that what you mean? I think that if the line is clearly straight then you can just assume that it is a line.

Hope this helps!!(8 votes)

- What is the difference between radians and degrees?(7 votes)
- Radians are a different way to measure angles.

I always remember there are 2*pi radians in a circle, which is also equals 360 degrees

So, one radian = 360 degrees/(2*pi) = 57.3 degrees

You'll use radians quite a bit in trigoinometry(3 votes)

- Accidentally used 90 degrees when solving it on my own and ended up with angle RPS = 5 degrees 😅 Make sure you use the right angle guys, and by 'right' I mean straight!!(6 votes)
- I'm told to determine the supplementary angle of a given edge with an improper fraction example 34 7/16 how do I go about that(4 votes)
- While your use of "a given edge" does not make any sense with determining supplementary angles, if your angle has a fraction such as 7/16, you subtract 180 - 34 = 146, then borrow one from that, so 145 + 1 - 7/16 = 145 + 16/16 - 7/16 = 145 9/16 degrees.(2 votes)

- l need help lm confuesed bc l still don't get algebra sir(4 votes)
- how can you solve for x(1 vote)

- How do you know if and angle is supplementary in a figure when it only has one found degree(3 votes)
- Supplementary means all angles add up to 180, right? Which means, if you know one degree (one angle) and there are 2 angles, then you know that the other angle is 180 minus the angle you already know.

For example, lets say that one angle is 20. There are two angles with a sum of 180. Since ONE angle is 20, the other MUST be 160, because that is the only thing that adds up to 180.

Equation format: x + y = 180

x = First angle that you are trying to find

y = Second angle you are trying to find

These two numbers MUST equal 180.(2 votes)

- i dont get it(3 votes)

## Video transcript

We're told that the measure
of angle QPR-- so that's this angle right over
here-- is 2x plus 122. And I'll assume that
these are in degrees. So it's 2x plus 122 degrees. And the measure of
angle RPS-- so that's this angle right over here--
is 2x plus 22 degrees. And they ask us to find
the measure of angle RPS. So we need to figure out
this right over here. So we would be able
to figure that out if we just knew what x is. And lucky for us, we
can use the information given to solve for
x and then figure out what 2 times x plus 22 is. And the main big idea here,
the thing that pops out here, is that the outside rays
for both of these angle form a line. These two angles form a line. You could say that
they are supplementary. Both of these angles
are supplementary. 2x plus 22 plus another 2x plus
122 is going to add up to 180. We know that this entire angle
right over here is 180 degrees. So we can say that the measure
of angle QPR, this angle right over here, 2x plus
122, plus the green angle, plus angle RPS-- so
plus 2x plus 22-- is going to be equal
to 180 degrees. And now we can start
simplifying this. We have two x's. We have another two x's. So those are going
to add up to be 4x. And then we have 122 plus 22. So that's going to be 144. And the sum of
those two are going to be equal to 180 degrees. We can subtract 144
from both sides. On the left-hand
side, we're just going to be left with a
4x, this 4x right here. And on the right-hand
side, we're going to have-- let's see,
if we were subtracting 140, we would have 40 left. And then we have to
subtract another 4, so it's going to be 36. Divide both sides by 4,
and we get x is equal to 9. Now remember,
we're not done yet. They didn't say solve for x. They said find the measure of
angle RPS, which is 2 times x plus 22 or 2 times
9 plus 22, which is 18 plus 22, which
is equal to 40. So the measure of angle
RPS is 40 degrees.