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Lesson 4: Missing angle problems

# Equation practice with complementary angles

Given the algebraic expressions that represent a pair of complementary angles, learn how to form and solve an equation to find an unknown angle. Created by Sal Khan.

## Want to join the conversation?

• how did you get 90
• how you know that it is a 90 degree is you will have the angle have a box at the part where it folds
• after 1.30 i got lost. i have no clue how he finished this equation. Specifically the part where he found out what x was.
• At , How do i know whether I should Add or subtract?
• We want to solve for x, and that means we need to isolate x on one side of the equation. Since we have 5x+40 = 90, in order to just have x on the left-hand side, we would need to get rid of the 5 and +40. First, +40. To get rid of it and make it 0, what do we need to do? Well, we need to subtract 40 since 40-40 = 0. But if we only did that to one side, the equation wouldn't hold up anymore. So, we subtract 40 from 90: 90-40 = 50. So, we have 5x = 50. And the left-hand side is still multiplying x by 5, so what do we need to do to get rid of that 5? Well, the opposite of multiplication is division. So, we divide by 5. 5 times x divided by 5 = x. But we also need to do the same operation to the other side. So, 50 divided by 5 = 10. And so we're left with x = 10.
• I might be a bit slow or dumb but i don't understand anything after 2x + 46 + 3x - 6 = 90 can you explain please.
• x in this case is a variable, and it could be any number, even a negative number or a repeating decimal like 2.334334...
So first we have 2x + 46 + 3x - 6 = 90
We add the Xs and the numbers on the left hand side to get
5x + 40 = 90
We subtract 40 from both sides to get
5x = 50
Finally, we divide both sides by 5
x = 10
Hope this helps
• Is ray the same thing as line?
• No, a ray is a like a line segment, and an arrow so it's basically an arrow on one end of a line and a dot on the other Ex. •-->

A line segment is a line with two dots on the sides. Ex. •--•

A line is a straight line that has arrows on the side. Ex. <---> Hope I helped you!
• Is anyone else confused by this? I mean like, yeah this is easy to most but i'm lost. Can i get help?
• just substitute complementary for "adds up to 90" and supplementary for " adds up to 180" in an equation. it should make sense then.
• What the heck is this
• Math
(1 vote)
• How did you gonna know which angle is 90 or the number
• Angles that are 90 degrees are marked with a little symbol that looks like the outer edges of a square, whenever you see that it means the angle is 90 degrees.