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Decomposing shapes to find area (add) | Math | 3rd grade | Khan Academy

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    - [Voiceover] What is
    the area of the figure?
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    So down here we have this one, two, three,
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    four, five, six, seven,
    eight, nine, 10-sided figure,
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    and we want to know its area,
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    how many square meters
    does this figure cover?
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    And we have some measurements,
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    that seems helpful,
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    but what's not too helpful to me is
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    I don't know the special trick
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    to find the area of a 10-sided figure
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    so I've got to think about what I do know
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    and what I do know
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    is the way to find area of a rectangle.
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    So what I can do, because I can see,
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    if I can find any rectangles in here.
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    Here's one rectangle, right there.
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    So I can find the area of that part.
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    Then let's see if I can find any more.
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    Here's another rectangle.
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    So I can find the area of that part.
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    We could call that one
    a rectangle or a square.
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    And then that leaves
    us with this last part,
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    which is again, a rectangle.
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    So what we did is, we broke this up
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    or decomposed it into three rectangles
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    and now if I find out how much space
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    this purple one covers,
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    and the blue one and the green one,
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    if I combine those, that
    would tell me the area
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    of the entire figure,
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    how much space the entire figure covers.
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    So let's start with this one right here.
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    This one is three meters long,
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    so we can kind of divide
    that by three meters,
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    into three equal meters,
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    and then we've got a width
    of two meters down here
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    so we can split that in half.
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    So if we draw those lines out,
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    we can see this top row is
    going to cover one square meter,
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    two square meters, three square meters,
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    and then there's two rows of that,
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    so there's two rows
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    of three square meters
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    for a total of six square meters.
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    This rectangle covers six square meters,
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    so this part of the entire figure
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    covers six square meters.
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    The next one, our measurements
    are three and a three,
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    so it will have three rows
    of three square meters
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    or nine square meters,
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    and then finally this purple one
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    has three meters and nine,
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    so we can say it will
    have three rows of nine
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    or nine rows of three square meters
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    which is 27 square meters.
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    So the area
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    of this purple section,
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    it covers completely 27 square meters.
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    The green covers nine square meters,
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    and the blue covered six square meters.
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    So, if we combine all those areas,
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    all those square meters it covers,
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    that will tell us the
    area of the entire figure.
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    So we have six square meters,
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    plus nine square meters,
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    plus 27,
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    and we can solve that,
    six plus nine is 15,
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    15 plus 27, let's see,
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    five ones and seven ones
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    is 12 ones.
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    We'll just find some space up there.
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    And
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    one 10 and two 10's
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    or a 10 and a 20
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    is 30.
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    And 30 plus 12 is 42.
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    So the area of the entire figure is
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    42
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    square
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    meters.
Title:
Decomposing shapes to find area (add) | Math | 3rd grade | Khan Academy
Description:

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Video Language:
English
Duration:
03:36

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