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# Tangents of circles problem (example 1)

Sal finds missing angles using the property that tangents are perpendicular to the radius.

## Want to join the conversation?

• So the opposite angles of a quadrilateral are supplementary.
Would that be proof of the first answer? Or is the first answer proof of that fact? Because I'll be honest, I try not to just jump to the easy answer when I'm learning a new concept - but the answer to the first question was practically reflex.
(3 votes)
• The opposite angles of a quadrilateral are not necessarily supplementary. The only condition on the angles is that all four of them must sum to 360˚.

If the opposite angles do happen to be supplementary, however, some interesting properties arise. One such property is that such a quadrilateral must be cyclic (that is, it must be inscribable in a circle).
(6 votes)
• couldn't you just to 92+x=180 and not do the extra steps when you do 92+180+x=360?
(2 votes)
• Yes, you can, but you have to make sure you don't mess up your information when you make this shortcut.
the extra information helps you understand how the problem works.
I hope this helps. :)
(7 votes)
• How did you get that 50 on that sec problem
(2 votes)
• Because since the degree measure of a minor arc is the same as its central angle, and Sal's central angle was 100, Arc BC was = 100. And since an inscribed angle's degree measure is 1/2 of the minor arc it intercepts, and D intercepts Arc BC, D = 50.
(5 votes)
• What does it mean when a line is "tangent to a circle?"
(2 votes)
• That it touches the circle at one and only one point. With this, it can be shown that a radius drawn to the point of tangency will be perpendicular to the line of tangency.
(4 votes)
• so at what's basically happening is that an angle at the center of the circle is double the angle made anywhere else on the circle, and conversely the angle anywhere else on the circle would be double the angle at the angle at the center.
(2 votes)
• Yes you are correct. It is a theorem, the inscribed angle is half of the central angle if and only if they are on the same arc.
(3 votes)
• instead of making it long a little long can I use the statement : "the opposite angles in a quadrilateral are equal to 180" so I can directly subtract 80 from 180 which will give me 100, which is angle COB.
(2 votes)
• not all quadrilaterals will have opposite angles equal to 180. but you can use the angle sum property of quadrilaterals is 360.

here, the radii make 90 degree angles with the tangents thus, the opposite angles make 180.
(2 votes)
• Is this stuff on the S.A.T.?
Cuz like I'm taking it before 7th grade and I have no clue whether to study this stuff cuz i never took high school :)
(2 votes)
• There is a bit geometry on the SAT, yeah. Most of it is algebra and "data analysis"/charts, but a few geometry questions will be there.
(2 votes)
• can someone explain the word "circumscribed" again? I don't really understand...
(1 vote)
• Circumscribed means, "drawn around", if I were to say a triangle was inscribed in a circle, the triangle would be drawn INSIDE the circle. But if I were to say a triangle was CIRCUMSCRIBED around a circle, this time the circle would be drawn INSIDE the triangle.

Hope this helped :D
(4 votes)
• why the tangent line is different the other line
(2 votes)
• if you understand it everything is fun
(2 votes)

## Video transcript

- [Voiceover] So we're told that angle A is circumscribed about circle O. So this is angle A right over here, we're talking about this angle right over there. And when they say it's circumscribed about circle O that means that the two sides of the angle they're segments that would be part of tangent lines, so if we were to continue, so for example that right over there, that line is tangent to the circle and (mumbles) and this line is also tangent to the circle. So you see that the sides of angle A are parts of those tangents, and points B and point C are where those tangents actually sit on the circle. So given all of that they're asking us what is the measure of angle A, so we're trying to figure this out right here and I encourage you to pause the video and figure it out on your own. So the key insight here and there's multiple ways that you could approach this is to realize, that a radius is going to be perpendicular to a tangent or a radius that intersects a tangent is going to be perpendicular to it. So let me label this, so this is going to be a right angle and this is going to be a right angle. In any quadrilateral, the sum of the angles are going to add up to 360 degrees, and if you wonder where that comes from well you can divide a quadrilateral into two triangles, where the sum of all the interior angles of a triangle are 180 and since you have two of them, it's 360 degrees. So this is 92 + 90 + 90 + question mark is going to be equal to 360 degrees. So let me write that down. 92 plus 90 plus 90 so plus 180. Plus, let's just call this X. X degrees. Plus X, they all have to add up to be 360 degrees so let's see, we could subtract 180 from both sides and so, if we do that we would have 92 plus X is equals to 180 and if we subtract 92 from both sides we get X is equals to let's see 180 minus 90 would be 90 and then we're just going to subtract two more so it's going to be X is equals to 88. So the measure of angle A is 88 degrees. Now let's do one more of these. These are surprisingly fun. All right. So it says, angle A is circumscribed about circle O, we have seen that before in the last question and they said what is the measure of angle D. So we want to find, let me make sure I'm on the right layer. We want to find the measure of that angle and let's call that, let's call that x again. So what can we figure out? Well just like in the last question we have a quadrilateral here, quadrilateral A, B, O, C and we know two of the angles, we know this is going to be a right angle we have a radius intersecting with a tangent, or part of a tangent I guess you could say. And then this would be a right angle and so by the same logic as we saw in the last question, this angle, plus this angle, plus this angle, plus the central angle are going to add up to 360 degrees. So let's call the measure of the central angle let's call that Y over there. So we have Y plus 80 degrees or we'll just assume everything is in degrees, so Y plus 80 plus 90 plus 90, so I could say plus another 180 is going to be equal to 360 degrees sum of the interior angles of a quadrilateral. And so let's see you have, we could have Y plus 80 is equal to 180, if I just subtract 180 from both sides if I subtract 80 from both sides, we get Y is equal to 100 or the measure of this is 100, the measure of this interior angle right over here is 100 degrees, which also tells us that the measure of this arc cause that interior angle intercepts... intercepts arc C, B, right over here, that tells us that the measure of arc C, B, is also 100 degrees. And so if we're trying to find this angle, the measure of angle D, that's an inscribed angle that intercepts the same arc. And we've seen in previous videos that an inscribed angle that intercepts that arc is going to have half the arc's measure. So if this is a 100 degree measured arc, then the measure of this angle right over here is going to be 50 degrees. So the measure of angle D is 50 degrees. And we are done.