# The Pythagorean theorem intro | Right triangles and trigonometry | Geometry | Khan Academy

In this video we’re going
to get introduced to the Pythagorean theorem,
which is fun on its own. But you’ll see as you learn
more and more mathematics it’s one of those cornerstone
theorems of really all of math. It’s useful in geometry,
it’s kind of the backbone of trigonometry. You’re also going to use
it to calculate distances between points. So it’s a good thing to really
make sure we know well. So enough talk on my end. Let me tell you what the
Pythagorean theorem is. So if we have a triangle, and
the triangle has to be a right triangle, which means that one
of the three angles in the triangle have to be 90 degrees. And you specify that it’s
90 degrees by drawing that little box right there. So that right there is– let
me do this in a different color– a 90 degree angle. Or, we could call
it a right angle. And a triangle that has
a right angle in it is called a right triangle. So this is called
a right triangle. Now, with the Pythagorean
theorem, if we know two sides of a right triangle we can
always figure out the third side. And before I show you how to
do that, let me give you one more piece of terminology. The longest side of a right
triangle is the side opposite the 90 degree angle– or
opposite the right angle. So in this case it is
this side right here. This is the longest side. And the way to figure out where
that right triangle is, and kind of it opens into
that longest side. That longest side is
called the hypotenuse. And it’s good to know, because
we’ll keep referring to it. And just so we always are good
at identifying the hypotenuse, let me draw a couple of
more right triangles. So let’s say I have a triangle
that looks like that. Let me draw it a
little bit nicer. So let’s say I have a triangle
that looks like that. And I were to tell you
that this angle right here is 90 degrees. In this situation this is the
hypotenuse, because it is opposite the 90 degree angle. It is the longest side. Let me do one more, just
so that we’re good at recognizing the hypotenuse. So let’s say that that is my
triangle, and this is the 90 degree angle right there. And I think you know how
to do this already. You go right what
it opens into. That is the hypotenuse. That is the longest side. So once you have identified the
hypotenuse– and let’s say that that has length C. And now we’re going to
learn what the Pythagorean theorem tells us. So let’s say that C is equal to
the length of the hypotenuse. So let’s call this
C– that side is C. Let’s call this side
right over here A. And let’s call this
side over here B. So the Pythagorean theorem
tells us that A squared– so the length of one of the
shorter sides squared– plus the length of the other shorter
side squared is going to be equal to the length of
the hypotenuse squared. Now let’s do that with an
actual problem, and you’ll see that it’s actually not so bad. So let’s say that I have a
triangle that looks like this. Let me draw it. Let’s say this is my triangle. It looks something like this. And let’s say that they tell us
that this is the right angle. That this length right here–
let me do this in different colors– this length right
here is 3, and that this length right here is 4. And they want us to figure
out that length right there. Now the first thing you want to
do, before you even apply the Pythagorean theorem, is to
make sure you have your hypotenuse straight. You make sure you know
what you’re solving for. And in this circumstance we’re
solving for the hypotenuse. And we know that because this
side over here, it is the side opposite the right angle. If we look at the Pythagorean
theorem, this is C. This is the longest side. So now we’re ready to apply
the Pythagorean theorem. It tells us that 4 squared–
one of the shorter sides– plus 3 squared– the square of
another of the shorter sides– is going to be equal to this
longer side squared– the hypotenuse squared– is going
to be equal to C squared. And then you just solve for C. So 4 squared is the same
thing as 4 times 4. That is 16. And 3 squared is the same
thing as 3 times 3. So that is 9. And that is going to be
equal to C squared. Now what is 16 plus 9? It’s 25. So 25 is equal to C squared. And we could take the positive
square root of both sides. I guess, just if you look at
it mathematically, it could be negative 5 as well. But we’re dealing with
distances, so we only care about the positive roots. So you take the principal
root of both sides and you get 5 is equal to C. Or, the length of the
longest side is equal to 5. Now, you can use the
Pythagorean theorem, if we give you two of the sides, to figure
out the third side no matter what the third side is. So let’s do another
one right over here. Let’s say that our
triangle looks like this. And that is our right angle. Let’s say this side over here
has length 12, and let’s say that this side over
here has length 6. And we want to figure out this
length right over there. Now, like I said, the first
thing you want to do is identify the hypotenuse. And that’s going to be the side
opposite the right angle. We have the right angle here. You go opposite
the right angle. The longest side, the
hypotenuse, is right there. So if we think about the
Pythagorean theorem– that A squared plus B squared is
equal to C squared– 12 you could view as C. This is the hypotenuse. The C squared is the
hypotenuse squared. So you could say
12 is equal to C. And then we could say that
these sides, it doesn’t matter whether you call one of
them A or one of them B. So let’s just call
this side right here. Let’s say A is equal to 6. And then we say B– this
colored B– is equal to question mark. And now we can apply the
Pythagorean theorem. A squared, which is 6 squared,
plus the unknown B squared is equal to the hypotenuse
squared– is equal to C squared. Is equal to 12 squared. And now we can solve for B. And notice the difference here. Now we’re not solving
for the hypotenuse. We’re solving for one
of the shorter sides. In the last example we
solved for the hypotenuse. We solved for C. So that’s why it’s always
important to recognize that A squared plus B squared plus C
squared, C is the length of the hypotenuse. So let’s just solve for B here. So we get 6 squared is 36,
plus B squared, is equal to 12 squared– this
12 times 12– is 144. Now we can subtract 36 from
both sides of this equation. Those cancel out. On the left-hand side we’re
left with just a B squared is equal to– now 144
minus 36 is what? 144 minus 30 is 114. And then you
subtract 6, is 108. So this is going to be 108. So that’s what B squared is,
and now we want to take the principal root, or the
positive root, of both sides. And you get B is equal
to the square root, the principal root, of 108. Now let’s see if we can
simplify this a little bit. The square root of 108. And what we could do is
we could take the prime factorization of 108
and see how we can simplify this radical. So 108 is the same thing as 2
times 54, which is the same thing as 2 times 27, which is
the same thing as 3 times 9. So we have the square root of
108 is the same thing as the square root of 2 times 2
times– well actually, I’m not done. 9 can be factorized
into 3 times 3. So it’s 2 times 2 times
3 times 3 times 3. And so, we have a couple of
perfect squares in here. Let me rewrite it a
little bit neater. And this is all an exercise in
simplifying radicals that you will bump into a lot while
doing the Pythagorean theorem, so it doesn’t hurt to
do it right here. So this is the same thing as
the square root of 2 times 2 times 3 times 3 times the
square root of that last 3 right over there. And this is the same thing. And, you know, you wouldn’t
have to do all of this on paper. You could do it in your head. What is this? 2 times 2 is 4. 4 times 9, this is 36. So this is the square root of
36 times the square root of 3. The principal root of 36 is 6. So this simplifies to
6 square roots of 3. So the length of B, you could
write it as the square root of 108, or you could say it’s
equal to 6 times the square root of 3. This is 12, this is 6. And the square root of 3,
well this is going to be a 1 point something something. So it’s going to be a
little bit larger than 6. ## 76 thoughts on “The Pythagorean theorem intro | Right triangles and trigonometry | Geometry | Khan Academy”

1. Maroof 4you says:

Sir nice vedio

2. Shirley H says:

Pythagorean Theorem is hard

3. Chinggis McKnight says:

10.4 is the length of that side (B)

4. Root InThePot says:

This video really helped me!! Thank you so much!!! They have practices too, it's in the description if you wanna try it.

5. Christian Smith says:

I swear I hate school😩😩 I just want a job😂😂😂😂

6. Isabella Someone says:

It's so weird to see math in a different language, you guys even switch the numbers lol

7. Michael says:

8:09 to 8:10 you almost said sex

8. Marwan Tamer says:

this makes math (dare I say it ) easy

9. chloe quach says:

well, i have SSAT, so i guess this is helpful. it's such a pain when i can be watching some random 5-minute crafts videos :,)

10. gdapangilinan says:

2019 hands off!

11. Dank Spank says:

Thank you I needed to remind myself how to do this @ 2:21 A.M.

12. J.D. Martinez says:

#YouCanLearnAnything

13. J.D. Martinez says:

#VideoGames

14. J.D. Martinez says:

#Top10Gaming

15. Little Nicky says:

This guy like to hear himself talk

16. Candice says:

Why is this thought in older years. It seems straight forward. I'm in 7th grade/ year 8 and algebraic expressions and indices seem harder. 🤦🤷

17. 2015 - 2019 says:

Too easy.

18. Erika Rodriguez says:

Why was the last 3 excluded ?

19. Chilufya Matipa says:

Good teacher. Nice voice

20. Tameem says:

What year level is this I’m in year 8 and need to study this

21. Clonify says:

This is so dumb 🧟‍♀️🖕

22. Leland Suchan says:

Give me your talent. I would love this for math! Like this comment if you agree!

23. Amybel Quinones Rodriguez says:

Oh yeah and sees🙂📓📕📔📒📗📚📚📖📘📙📝🔎1000

24. Shannon Chuprevich says:

Isn't the theorem a form of proofing pie?

25. Kazia Inku says:

Nice video

26. G - says:

Have my finals for math tmr 😰

27. The Piggy Power says:

Its pretty ez thx

28. Hey it's Em says:

Never underestimate the power of khan academy. I have a final exam tomorrow and I was crying earlier cuz I didn't remember or understand how to do all this stuff. now I've watched these videos I'm a pro. literally god bless khan academy you saved my sanity

29. Aguilera Productions says:

I’m here because I heard Daniel Habif mention it in one of his videos. Is there something spiritual to it????

30. Defined Dragon says:

I was absent during this and I have my final today, thx, also your handwriting is really good

31. Cohen B. says:

when you teach yourself the entire course the night before the exam lol. wish me luck, kids

32. Icy says:

Him: 16 + 9 = 25
Me: YES THATS SO EASY
Him: We get the roots
Me: Quits

33. GamingWithEve _h2o says:

34. Dain Kim says:

I've got a courage to master math, with khan.
Thank you for your nice math videos and I encourage you being a math teacher of
math
in South Korea's American math
academy and you would be a [email protected]!

35. Hyper_z11 says:

The best thing is that these videos have a black background which helps in watching in the dark

36. Bridget Dorothy Lucio says:

It's school break but I'm still studying. ;-;

37. Keda says:

Somehow I'm learning more effectively because of these videos

38. Raid Area 51 says:

8:59 this guy is a legend, me getting square root uses calculator

39. Cvh Typical says:

Don’t forget translate to arabic please

40. Mari Christian says:

That was fun. Thank you!

41. Hitanshi Gaudani says:

Math is a drama queen, it can’t seriously have that many problems!

42. Zoe Sherman says:

This is why I homeschool

43. miscellaneous • stuff says:

who else is studying for the SHSAT?

44. Urja Jain says:

Thank you so much! This has been impossible for the longest time, and now I feel so confident ☺️

45. Ocean love says:

Teacher: Triangle class, studie

Students: huh??

Edit: I’m in 5th grade so I don’t know what the heck is going on.

46. Cedar Poplar says:

So my question is: Why is it called a therum? Numbers are factual. Aren't they??

47. Muhammad Ahmed Abbas says:

Thanks Sal! You really helped

48. Just Dextii says:

PIE TAH GO REE YAN TYO REM

49. Jred8282 says:

c=5
Why do you get 5?

50. X-RAY ASSA55IN says:

can someone explain 10:01, why is he able to take the last three off? does he have to for the equation to work? please help.

51. Fated Mite says:

Angel???

52. Vishnu Darsan says:

Mathematics is so funnnn!!! U make it seem so easy! Respect sal!!!!

53. Jimmyzb36 says:

The Pythagorean Theorem, woke me? When I went to technical school! Electronics. It Works, proves, and ___________, Fill in the blank.

54. D L S says:

Terrance Howard sent me hear SEP ”19”

55. NotFunnyHuman says:

Thank you! I was really struggling with a worksheet in math but now I completely get it.

56. thunder says:

Tysm, I was trying some problems involving this using just my math textbook and i got it super wrong and this helped explain it

57. AllMight is best girl says:

thank you for helping me with my math stuff fellow human

58. Ibrahim Abdullah Bokhari says:

I remember watching this 7 yers ago, and now I’m watching it again taking note on how to teach PYTH. THEO. to tutor middle school kids tomorrow. Idk how much I owe you from learning everything from you, ASTRONOMY, PHYSICS, CHEMISTRY, MATH, DUDE THEY GOT EVEN MUSIC lessons TOOO!!

59. Josh Hanequiez says:

I came because of curiosity

60. Arsema H says:

This is the best .It really helps to understand its concept and is easy to understand. for those of you who haven't seen it yet check it out.

61. Kody Bauer says:

When I was in 8th grade I jumped into 9th grade Algebra 1B 8th learned this, I never did. Have never needed this in math. I'm in 9th grade now and am currently taking 11th math and 10th science and English

62. Kili kachuo Naoro says:

Thank you so much!! I've never taken Geometry but you actually made it look easy

63. Dafe Snow says:

Sounds like
Pytha korean

64. Gicho Georgiou says:

It was going well……until the last 2 minutes, then I felt like a Japanese bomber nose diving into the sea

65. Griffin Gibson says:

November 3, 2019

66. Unknown User says:

well explained 👏
goodjob 👏

67. Jesse Brown says:

Good luck kids

68. andriki88 says:

I wish I was high on potenuse

69. Tharanga Harshani says:

His computer handwriting is better than my normal handwriting

70. SuperSonicKazooie says:

Thumbs up if you remember the Pythagorean theorem problem from your algebra class in college just like I did! 😀

71. jana nn says:

Arabic translation?!

72. Lamya Mcdonald says:

Math was fun before they put the alphabet in it 😢😂
Btw thanks who knew it was actually this simple

73. Sushi? Sushi!! says:

Cool! I still don’t understand!

74. RAVEN GILES says:

uh hu and tell me how to solve 7×2+x2=(x=1)2

75. Clout Monster says:

76. James Ryerson says:
77. Gplayminion says: