Central Angle Theorem

blackpenredpen,
math for fun,
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[@Pamekimo]

minus Y, plus Y also say bye bye

[@General12th]

+

[@alitrux]

I subscribed for blackpenredpen, not for blackpenredpenbluepen. I am very dissapointed.

[@General12th]

I think redpenblackpen is _way_ better than blackpenredpen.

[@blackpenredpen]

Alitrux i am sorry........ : )

[@williamtachyon2630]

In 2019, it may be that his name will be blackpenredpenbluepenyellowpengreenpenorangepenpurplepen.

[@VerSalieri]

I never understood that blackpenredpen thing, why not just use blackredpen^2? It just doesn’t compute to me..

[@TheYoshi463]

Is it commutative though?

[@chesteezy5197]

My Number Theory professor disagreed with the theorem after I told him what it was. LOL

[@balajishankar]

Fantastic.....especially the second part of the proof!

[@blackpenredpen]

Thanks!!

[@Klayx_]

night before maths exam. this man is amazing

[@akshat9282]

Instead of saying that taking a point in the minor arc would be no good, you could have just stated why. It's a basic explanation and you just subtract both angles by 360°. Btw I had this in my curriculum 5 years ago and I still haven't forgotten this beautiful theorem wow good job bprp

[@varunmurali5671]

Akshat K Agarwal can u elaborate your statement? I didn't understand subtracting 360 from the 2 angles (I also don't know which two angles you are talking about)

[@akshat9282]

Varun Murali sure. Say you're given that the angle of the minor arc AC is 60° and the point B is on the minor arc instead of the major arc. Then then angle ABC(the angle inside the circle) would be 150° since you can consider the central angle as convex. So it'll be 300° (because 360°-60°). And using the same theorem, the angle ABC becomes 150°

[@ingiford175]

The proof I like most is the one that starts with A&B on opposite sides of a diameter. Once you prove that, you can prove the other two cases you have quickly.

[@ThePowerfulOne07]

Love geometry theorems and concepts!

[@dishwasherdog]

I wish I was you brodi. Imma fail this Damn test

[@holyshit922]

We can use it for writing equation of angle bisector but it will be a little bit longer than comparing distances of point from both rays of angle

[@yaleng4597]

our school requires us to write angle at centre twice angle at circumference

[@ankaparnahajova]

Precisely my thinking, I like to show precisely these three variations. Thank you for the video.

[@MA-mp5mx]

Why are you keep this name blackpenredpen

[@gnikola2013]

You forgot the prove the case when the segment AB colinear to the segment AO!!

[@gnikola2013]

Thanks! lol

[@kkkkkkkk1179]

On that figure, OB=OA(radius) => OBA=OAB=x(let) 180°=AOB+OBA+OCA(angle sum property) =>180°=2x+AOB 180°=AOB+AOC(linear pair) On equating, 2x+AOB=AOB+AOC =>2x=AOC =>2AOB=AOC

[@Apollorion]

@Kiritsu Insert x=0 in the formula's on the left halve of the board and they describe the situation you suggest.

[@qqqquito]

Picking a point on the minor arc AC is still good. In which case, the central angle AOC is a "reflex angle", and the inscribed angle ABC is an obtuse angle and is half of the reflex angle AOC.

[@HWMREWesker]

Stuff from the 8:40 point is unnessecary. Angle AOB equals (180-2y+2x) from AOB triangle. Angle COB equals (180-2y) from COB triangle. Angle AOC equals angle AOB minus angle COB and that's (180-2y+2x)-(180-2y)=180-2y+2x-180+2y=2x, Q.E.D.

[@AndDiracisHisProphet]

crap, this is obvious -.- I feel embarrassed now

[@General12th]

Yeah, it actually makes way too much sense...

[@debrajbanerjee9276]

What is the integral of sqroot(sinx) from 0 to pi? I found it 2√(2/π)(gamma(3/4))^2 in wolfram alpha which seems very interesting.can you provide me the magical steps?

[@akshat9282]

Substitute tan(x/2) = u and you'll get everything after that

[@debrajbanerjee9276]

Akshat K Agarwal after this substitution the integral would be 2sqrt(2u/(1+u^2)^3) which would not work

[@akshat9282]

Debraj Banerjee what no. Differentiate the substitution. You'll get (1/2)sec^2(x/2)dx=du Then change the sec to tan if you want. I'm sure you can figure out what to do next if you talk about gamma in your integrations

[@akshat9282]

Debraj Banerjee also if you graph the function, you'll notice that it is symmetric at x=π/2 so computer the integral to that point maybe and then multiply by 2 because at π/2,you can use a property of definite integrals and change u from u to π/2 - u This is a higher level integral and I don't think he'd be able to cover it anytime soon. If you need help, you can still ask or Google it. All the best

[@akshat9282]

Hilbert Black yesss

[@thetophatgentleman4634]

The day blackpenredpen did simple things.

[@holyshit922]

I use central angle theorem for construction regular 2n gon after construction n gon with given side length

[@teekayanirudh]

I think until 5:45 was the essential part of the proof, since if we join A and C we get a chord AC of a circle and angles subtended by a chord of a circle on it are all equal. But I really like that you did the other case as well for completeness :)

[@randomname9291]

I’m pretty sure that that sentence derived from the central angle theorem and not vice versa

[@bilalabbad7954]

The geometry is incredible

[@Intskints]

Now it makes sense how Thales thm works, cause the diameter's central angle is 180 degrees.

[@UnathiGX]

Yay...I just learned a new trick..Thanks BPRP

[@chessandmathguy]

Beautiful.

[@blackpenredpen]

steamroller82 yay!!!

[@tangentofaj]

Hopefully you like this? I LOVE THIS.

[@wowfmomf6126]

Thanks for the daily doze of math

[@labyrinth6091]

When i was 13yo just solved it in like 20 sec, but now with 24 i couldn't. Is that bad?(don't know what to feel)

[@frederickm9823]

In the second case, why didn't you use the triangles OCB and AOC. The OCB one gives you the angle on the right side (180-2y) and the AOC one gives you the whole angle (180-2*(y-x)), so the angle we are looking for is: 180-2*(y-x) - (180-2y) = 180-2y+2x-180+2y = 2x Therefore the angle in the middle is twice as big as the one on the outside. Yet I liked your way, too, but it was a bit too complicated, I guess

[@typo691]

Hey can you tell me why that angle is y - x? EDIT: Nevermind, I understand now. It looked confusing because of the dimensions of the diagram, I forgot that it was isosceles.

[@typo691]

@Anurag MFT OC = OB so if OCB = y then OBC = y cause it's an isosceles triangle. Then it's easy to see that OBA = y - x

[@pipithandayani]

for the real life, what the situation we can explaint about this theorem?

[@purim_sakamoto]

へええ　これはみんな覚えとくと便利だねえ😄

[@darshnavijay4339]

Best explanation

[@General12th]

If I squint my eyes and tilt my head, your diagram kinda looks like a pentagram.

[@migtrewornan8085]

If point B coincided with point C wouldn't the angle ABC be zero. In that case angle ABC must approach zero as B approaches C and hence cannot be constant?

[@tigerbeast3406]

Point C is kinda like the border between the big AC arc and the small AC arc. Point B, as we know, must be on the big AC arc. The theorem doesn't work if point B is on the small arc or on its borders. If point B and point C are on the same place, the angle ABC is 0, but the theorem is not appliable for this case. Even if point B is very close to point C, as long as it is on the big arc, angle ABC is two times smaller than its corresponding central angle.

[@marcypan8219]

I FINALLY UNDERSTAND THIS!!!!!!!!!!!

[@sharnezhu2860]

let us peep the shoe game

[@JorgenJorgensenSonofJorgen]

Is there a way to prove this without splitting into multiple cases?

[@abhivish2017]

Nice explanation sir.....

[@Sid-ix5qr]

I was taught that Central Angle in a Circle was always 90°.

[@noorashfaq3978]

'semi circle...

[@gowrishi1899]

nice rhyme in 10:19

[@SuperSatyasrinivas]

Good

[@mdmuktadi7980]

Cool bro

[@billy.7113]

Why don't you just write ∠AOC = 2∠ABC instead of m∠AOC = 2m∠ABC ? m is confusing.

[@madamehajj2816]

What about interior angle

[@williamtachyon2630]

Great video, as always. I really love mathematics, isn't it! :)

[@andrewjustin256]

7:40 why is this angle y-x?

[@sergioavramescu]

Because it is an isosceles triangle, the angles OCB and OBC has to be the same

[@basilemaddalena1586]

Hi, can you figured out the derivative of n ! ?

[@hattorikanzo2793]

Yes

[@zhandexiang222]

how did the y-x come to be

[@xmatterx9246]

Yaaaayyyyyyy

[@kaaiplayspiano7200]

What?

[@retired5548]

i guess you can say that it's a central theorem for angles in a circle

[@notananimenerd1333]

K

[@DjVortex-w]

That's the Star Trek symbol...

[@BigDBrian]

but what if B is on the left side instead of the right side :^)

[@TheReaper9520]

You can get around that by slapping a WLOG onto the second scenario ::^^))))

[@stevethecatcouch6532]

Copy his proof on the inside of a window, then look at it from outside.

[@linh4010]

666 likes

[@nusratakeya2378]

nonsense