Function notation example

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Sal uses function notation to help Frank figure out how much water he can put in his balloon. Created by Sal Khan.

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Saba Ali
Posted 11 years ago. Direct link to Saba Ali's post “At 1:53 don't you multipl...”
At
1:53
don't you multiply each of the terms by three to get 4 as a whole number, not divide?
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(7 votes)
Answer
- Carzo, Kainaza
  Posted 9 years ago. Direct link to Carzo, Kainaza's post “no because you would have...”
  no because you would have to multiply the other side too and if you did that you would get the wrong answer
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  (3 votes)
Adeola Boye
Posted 9 years ago. Direct link to Adeola Boye's post “why did he divide 27 by 3...”
why did he divide 27 by 3 at
1:47
instead of 4 divided by 3?
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(12 votes)
Answer
- Stefen
  Posted 9 years ago. Direct link to Stefen's post “You are free to do it in ...”
  You are free to do it in which ever order is easier for you (as long as you observe PEDMAS).
  In this example we have 3?=27 and 4/3.
  If you did 4/3 first, then you would have to work with 1.33333333333 (inaccurate), or the mixed fraction "1 and 1/3" and multiply those by 27.
  
  So now you have to multiply 27 by 1.33333333333 or "1 and 1/3" that to get the answer. Can you do that in your head?
  
  Sal noticed that 27 is divisible by 3, that is 3x9=27, and with that he can simplify the expression by removing a factor of 3 from the 3 in 4/3 and 27 to get 4 and 9 to get 36 - easy to do in your head.
  
  You also could have taken the factor of 3 out of 3? and 4/3, to get 3? and 4 to get 9 and 4 to get 36 - also easy to do in your head.
  
  So you can do it in the order that is easiest for you - the goal is always to reduce careless errors.
  Comment on Stefen's post “You are free to do it in ...”
  (17 votes)
amvasquez102
Posted 9 years ago. Direct link to amvasquez102's post “besides just water balloo...”
besides just water balloons, where does function notation come in handy?
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(9 votes)
Answer
- moomoosnake
  Posted 9 years ago. Direct link to moomoosnake's post “Function notation in math...”
  Function notation in maths is analogous to the list of ingredients you get given in a recipe. It won't tell you how to make the cake but it will tell you what ingredients you'll need to get when you go shopping! It's used practically in physics, and is one of the key elements in computer programming. It's particularly useful when you're looking at messy relationships with multiple variables. as it provides a quick idea of which variables are important in determining something's value.
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  (10 votes)
James Mugo
Posted 10 years ago. Direct link to James Mugo's post “How do you compute the ar...”
How do you compute the area of a rectangle as a function of it's width given its perimeter.
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(6 votes)
Answer
- Qeeko
  Posted 10 years ago. Direct link to Qeeko's post “Let w denote the width an...”
  Let w denote the width and h the height of the rectangle in question. Given the perimeter P, we may write P = 2w + 2h. Hence h = P/2 - w. The area as a function of its width is then given by
  A(w) = wh = w(P/2 - w).
  Comment on Qeeko's post “Let w denote the width an...”
  (9 votes)
Nneka
Posted 10 years ago. Direct link to Nneka's post “I'm not sure if I'm right...”
I'm not sure if I'm right...
So since this is a function, would the radius be the input/domain?
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(8 votes)
Answer
evelyn.lai
Posted 4 years ago. Direct link to evelyn.lai's post “hi, can someone help me w...”
hi, can someone help me with this problem I got? I don't know why my answer was wrong.
f(x)=-x-4. find f(-4)
the answer options were
a.0 (this was the correct answer)
b.-8 (this was the answer I got)
c. 4
d. 8
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(4 votes)
Answer
- Savraj Sahota
  Posted 4 years ago. Direct link to Savraj Sahota's post “In this case, you would s...”
  In this case, you would simply need to plug in -4 into the equation. So, f(-4)=-(-4)-4. This equals zero because you have a negative times -4, which equals positive 4. Then, you subtract 4, which equals 0! I think you forgot about the negative. Let me know if this helps!
  Comment on Savraj Sahota's post “In this case, you would s...”
  (5 votes)
Yusef Gadson
Posted 5 years ago. Direct link to Yusef Gadson's post “I need help with evaluati...”
I need help with evaluating functions
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(5 votes)
Answer
Viola
Posted 8 years ago. Direct link to Viola's post “At 2:08, why doesn't Sal ...”
At
2:08
, why doesn't Sal solve 36(3.14)? Why does he leave it as 36Pi?
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(4 votes)
Answer
- Faith Gbeneneh
  Posted 4 years ago. Direct link to Faith Gbeneneh's post “Well, pi is a number that...”
  Well, pi is a number that goes on and on and in that case, we won't be able to get the exact number for pi. So, using 36pi would be more exact than using 36(3.14).
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  (2 votes)
testsubjectmike420
Posted 5 months ago. Direct link to testsubjectmike420's post “my brain does not underst...”
my brain does not understand this pls help me and my brain
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(4 votes)
Answer
Sannuth Rao
Posted 10 years ago. Direct link to Sannuth Rao's post “Why do you not also multi...”
Why do you not also multiply Pi in 4/3 Pi r cubed. When the answer of the question gets to 4/3 Pi 27, why do you not have to multiply Pi with 27 and 4/3 in the problem. Is the problem solving for Pi
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(4 votes)
Answer
- ?＿?
  Posted 10 years ago. Direct link to ?＿?'s post “Pi is an irrational numbe...”
  Pi is an irrational number, and thus cannot be solved for all the way- It goes on forever without a pattern and cannot be represented as a fraction. The answer is impossible to get without rounding (inaccurate) or leaving the pi out. Thus, in order to avoid confusing the watcher by providing a non-simple or incorrect number, he chose not to multiply pi into the other values.
  Comment on ?＿?'s post “Pi is an irrational numbe...”
  (1 vote)

Video transcript

Frank wants to fill up a spherical water balloon with as much water as possible. The balloons he bought can stretch to a radius of 3 inches-- not too big. If the volume of a sphere is-- and this is volume as a function of radius-- is equal to 4/3 pi r cubed, what volume of water in cubic inches can Frank put into the balloon? So this function definition is going-- if you give it a radius in inches, it's going to produce a volume in cubic inches. So let's rewrite it. Volume as a function of radius is equal to 4/3 pi r cubed. Now, they say the balloons he bought can stretch to a radius of 3 inches. So let's think about, if the radius gets to 3 inches, what the volume of that balloon is going to be. So we essentially would just input 3 inches into our function definition. So everywhere where we see an r, we would replace it with a 3. So we could write-- and just to be clear, let me rewrite it in the same color. V of-- that's not the same color. We do it in that brownish color right over here. So V of 3 is equal to 4/3 pi-- and instead of r cubed, I would write 3 cubed-- 4/3 pi 3 cubed. This is how the function definition works. Whatever we input here, it will replace the r in the expression. So V of 3 is going to be equal to-- so this is going to be equal to 4/3 pi times-- 3 to the third power is 27. 27 divided by 3 is 9, so this is 9. 9 times 4 is 36 pi. So this is equal to 36 pi. And since this was in inches, our volume is going to be in inches cubed or cubic inches. So that's the volume of water that Frank can put in the balloon-- 36 pi cubic inches.