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# 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?

• 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.
• Haha! Yes these videos can get confusing. He was trying to figure out what X was. To do that, you need to isolate it. First combine like terms (The ones with X, 2x and 3x, and the ones without X, 46 and -6.). That's how he got the equation 5x+40=90. Next you subtract 40 from both sides. Now you have 5x=50 (because 90-40 equals 50). To get rid of the 5, you divide it from both sides. You have X! x=10!
• 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
• 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
• 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.
• Maybe this question is a little bit out of topic, but I am wondering:
We "denote" a ray with an arrow on top of the expression, but is this in any way related to vectors? (Vectors are beyond of what I've learned so far, but I've seen the notation multiple times in that context)