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Do volume and capacity
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mean the same thing?
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The answer is no.
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There's a very subtle difference between them.
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In order to understand that,
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let me give you a simple example.
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Let us take a wooden box
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open from the top.
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It has some thickness ofcourse.
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The space occupied by the box
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is its volume.
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And the space available for you
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to keep something inside
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is its capacity.
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If I keep increasing
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the thickness of the box,
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you can easily see
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that the space inside it
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keeps decreasing.
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Or in other words
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its capacity is decreasing.
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But it's volume remains the same.
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If I keep increasing the thickness,
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the box will become fully solid
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and there'll not be
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any space available inside it.
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This means it has zero capacity.
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But still the volume
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remains the same.
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This means that any object
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will have volume but it may
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or may not have a definite capacity.
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So volume is the space
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occupied by an object,
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whereas capacity refers to the
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ability of the object
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to contain something.
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Let's quickly summarize
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what we've seen so far.
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The shapes that we've seen
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and what the surface areas
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and volumes for
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each of these shapes are. Right.
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The entire chapter,
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all of surface area and volume
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is nothing but this 1 table
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that we'll now see.
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Let's start with the
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first shape that we saw.
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Cuboid with dimensions
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length*breath*height.
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How will you find
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the total surface area
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of this cuboid?
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Let’s first find
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the area of the flat faces.
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The cuboid has only flat faces.
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Let me open up the cuboid.
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What do you see here?
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You see 6 rectangles.
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2 of them
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have area length*breadth.
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2 of them have area
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breadth*height.
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2 of them have area
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length* height. Right.
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So the surface area,
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the flat surface area
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is going to be
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2(length*breadth+breadth*height+
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length*height).
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The curves surface area is zero.
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The cuboid does not have
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any curved surfaces.
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So the total surface area
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will be same as
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the flat surface area
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that we saw.
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Coming to the volume,
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for the volume
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think of 1 rectangular plate
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with length=L
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and breadth=B.
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Now stack up
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H such plates. Right.
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So what is a total volume
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you’ll get?
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1 plate had an area
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length * breadth.
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H such plates.
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So length * breadth * height.
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That is nothing but the
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volume of your cuboid.
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Next figure that we saw was a cylinder.
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Let's take a cylinder with
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radius r and height H.
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This is the cylinder that you have.
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It has 2 flat faces. Right.
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If we open this out
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this is nothing but a circle
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and this is a circle. Right.
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The area will be
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πr² and πr²
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Total flat surface area
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will be 2πr²
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Now the remaining portion
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of the cylinder is
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nothing but the curved portion.
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Let me cut it here
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and open it up.
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What do you see?
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Nothing but a rectangle.
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This length
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is nothing but 2πr. Correct.
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Because it was a circle originally.
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It was a circle. So it's 2πr
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When we open it up it remains the same.
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So this is 2πr
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and this H.
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Area of this rectangle will be
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2πrh.
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This is nothing but the
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curved surface area
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of the cylinder. Right.
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Total surface area,
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just add the 2.
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2πr²+2πrh. Right.
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So you’ll get 2πr(r+H).
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That's the total surface area.
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For the volume,
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what is a cylinder really?
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If you take 1 circular plate.
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Right. Of radius H
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and stack up
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H such plates,
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you'll get a cylinder. Right.
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You'll get a cylinder.
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So volume is nothing but
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this area πr²*H because you have
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H such plates. Right.
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That's the volume of your cylinder.
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πr²H.
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The next figure
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is a cone.
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Let's take a cone
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with radius of base r,
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height H
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and slant height L. Right.
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Slant height L.
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The flat surface
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that you see here is
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this circle at the bottom.
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If you remove this,
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this is nothing but a circle.
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Area πr².
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That's the flat surface area.
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The remaining portion of the cone
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we have left is
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the curve surface area.
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If I now cut it from here
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and open it out,
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what you'll have
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is the sector of a circle,
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with this radius = L
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and this length
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of the sector = πr. Right.
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If you add these up
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the total surface area will be
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πr*r+L.
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Now volume of a cone.
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If you take a cylinder
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we already know the
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volume of a cylinder.
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Now take a cone
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with same radius of base
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as cylinder and same height
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as cylinder.
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If we pour water
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from the cone to the cylinder,
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you will see that
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you'll require to pour it
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3 times to fill the cylinder.
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Which tells you,
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volume of the cone is
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1/3 the volume of the cylinder.
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Right.
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So volume of cone will be
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1/3πr²H.
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Let's take a sphere now.
190
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If you look at this
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the sphere has
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no flat surfaces. Right.
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So flat surface area
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will be 0.
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It only has curved surfaces.
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If you measure the
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curve surface area of a sphere,
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it will be the same as
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the surface area of
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4 circles
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of the same radius. Right.
202
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So if we have
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this sphere of radius r,
204
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we'll take 4 circles
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of the same radius r,
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the area of these 4
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will be the same as
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area of the sphere.
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Right.
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So it's nothing but 4πr²
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Now the total surface area
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will remain the same 4πr²
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because it had no flat faces.
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If you take the volume,
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the volume of this sphere
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is nothing but 4/3πr³ Right.
217
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So the volume of this sphere
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will be 4/3πr³.
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That's it.
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The entire chapter is complete.
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All that you need to remember
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is this one table. Right.
223
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And once you know the visualisation
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you don't even need to
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learn this table. Right.
226
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Just remember the visualisation
227
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and you'll automatically know
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what the formula is.
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That's it.
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The chapter is really over.
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Just these 5 minutes
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and the chapter is over. Right.
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This is all that
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you need to know.
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Problems is nothing about
236
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putting numbers into these formulae,
237
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in some cases combining
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2 of these kinds of objects
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to find area.
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That's it.
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There is nothing else to this.
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Right.
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Just these 5 minutes
244
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and the chapter is done.
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After that the only thing
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you have to take care of is
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not making calculation mistakes.
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That's it.
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5 minutes and your
250
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chapter is complete. Right.
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One of the biggest chapters
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and 5 minutes and it's complete.
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Right.
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So always visualize in geometry.
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More importantly in 3 dimensional geometry
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it's very important to visualize.
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The moment you visualize
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you don't need to memorize
259
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any of this. Right.
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If you think of the visualization,
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you'll automatically know
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what the formulae need to be.
16188
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