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Median Centroid Right Triangle Example

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    So we're told that
    AE is equal to 12.
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    That's this side
    right over here.
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    And EC is equal to 18.
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    And then they've drawn a bunch
    of the medians here for us.
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    So we know that they are medians
    because, when they intersect
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    the opposite side, they're
    telling us that this length is
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    equal to this length, or
    that ED is equal to DC,
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    CB is equal to BA,
    AF is equal to FE,
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    or that F, B, and
    D are the midpoints
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    and that G, then, would
    be the centroid where
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    the medians intersect.
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    And so the first thing they ask
    us is, what is the area of BGC?
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    So BGC right here.
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    That is this triangle
    right over there.
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    And to figure out
    that area, we just
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    have to remind ourselves that
    the three medians of a triangle
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    divide a triangle into six
    triangles that have equal area.
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    So if we know the area of the
    entire triangle-- and I think
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    we can figure this out.
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    This is a right triangle.
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    They're telling us that.
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    AE-- this entire distance right
    over here-- is going to be 12.
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    So this is going to be 12.
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    Let me make sure I
    have enough space.
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    This entire distance
    right over here is 18.
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    They tell us that.
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    So the area of
    AEC is going to be
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    equal to 1/2 times
    the base-- which
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    is 18-- times the height--
    which is 12-- which
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    is equal to 9 times
    12, which is 108.
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    That's the area of this entire
    right triangle, triangle AEC.
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    If we want the area of BGC or
    any of these smaller of the six
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    triangles-- if we ignore
    this little altitude
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    right over here, the ones that
    are bounded by the medians--
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    then we just have
    to divide this by 6.
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    Because they all
    have equal area.
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    We've proven that
    in a previous video.
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    So the area of BGC is
    equal to the area of AEC,
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    the entire triangle, divided by
    6, which is 108 divided by 6.
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    Which is what?
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    It's 60-- let's see.
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    You get 10 and then 48.
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    Looks like it would be 18.
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    It would be 18.
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    And that's right
    because it would
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    be-- 108 is the same
    thing as 18 times 6.
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    So we did our first part.
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    The area of that right
    over there is 18.
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    And if we wanted,
    we could say, hey,
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    the area of any of
    these triangles--
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    the ones that are bounded
    by the medians-- this
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    is going to be 18.
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    This is going to be 18.
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    This entire FGE triangle
    is going to be 18,
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    but we did this first
    part right over there.
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    Now they ask us, what
    is the length of AG?
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    So AG is the distance.
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    It's the longer part of
    this median right over here.
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    And to figure out
    what AG is, we just
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    have to remind ourselves
    that the centroid is always
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    2/3 along the way
    of the medians,
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    or it divides the
    median into two segments
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    that have a ratio of 2 to 1.
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    So if we know the entire
    length of this median,
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    we could just take 2/3 of that.
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    And that'll give us
    the length of AG.
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    And lucky for us, this
    is a right triangle.
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    And we know that F and
    D are the midpoints.
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    So for example, we
    know this AE is 12.
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    That was given.
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    We know that ED is
    half of this 18.
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    So ED right over here--
    I'll do this in a new color.
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    ED is going to be 9.
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    So then we could just use
    the Pythagorean theorem
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    to figure out what AD is.
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    AD is the hypotenuse
    of this right triangle.
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    So we're looking at
    triangle AED right now.
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    Let me write this down.
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    We know that 12
    squared plus 9 squared
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    is going to be
    equal to AD squared.
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    12 squared is 144.
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    144 plus 81.
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    And so this is going to
    be equal to AD squared.
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    So this is what?
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    This is 225.
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    So we have 225 is
    equal to AD squared.
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    And 225, you may or may not
    recognize, is 15 squared.
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    So AD is equal to 15.
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    You want to take the principal
    root, the positive root,
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    because we're talking about
    distances or lengths of sides.
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    We don't care about
    the negatives.
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    So AD is equal to 15.
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    So this whole thing
    right over here
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    is going to be equal to 15.
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    And AG is going to be 2/3 of AD.
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    We proved that in
    a previous video,
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    that the centroid is
    2/3 along the way of any
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    of these medians.
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    And we could do it for
    any of the medians.
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    So it's equal to 2/3 times
    15, which is equal to 10.
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    So AG right over
    here is equal to 10.
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    So we did the second part.
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    Now, this third part,
    what is the area of FGH?
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    So let me color it in-- FGH.
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    So if we knew this length-- if
    we knew HG and if we knew FH--
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    we could easily figure
    out what that area is.
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    And there's actually
    multiple ways
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    of figuring out either
    one of those things.
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    So one way that we can think
    about finding what HG is
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    is to remind
    ourselves that HG is
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    the altitude of either
    triangle FGE or triangle AFG.
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    And both of them
    have a base of 6.
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    So this is 6, and
    this is 6 over here.
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    And then they have a
    height equal to GH.
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    And we know what the area is.
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    We know that the area
    is already equal to 18.
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    So let's take this
    triangle up here.
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    So we're talking about
    the area of triangle AFG.
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    So we know it's 1/2
    times its base, which
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    is 6, times its height,
    which is GH-- times GH.
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    That's just 1/2
    base times height
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    is equal to the area
    of this triangle, which
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    is going to be equal to 18.
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    And so then, we just have
    to tell ourselves, well,
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    this is-- 3 times
    GH is equal to 18.
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    If we divide both sides of
    this by 3, GH is equal to 6.
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    So that is one way to do it.
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    GH is equal to 6.
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    You could have also made
    the similarity argument.
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    Then you could have said,
    look, this triangle up here
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    is similar to this larger
    triangle over here.
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    This hypotenuse is 2/3 of the
    length of this entire thing.
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    So this is going to
    be 2/3 of this 9.
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    So that's another way that
    you could have gotten 6 there.
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    But either way, we
    got this length.
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    Now we just have to
    figure out what FH is.
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    And we could figure out what
    FH is if we know what AH is.
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    Because we know A to F is 6.
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    So FH is going to
    be AH minus AF.
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    So let's figure out what AH is.
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    Well, once again, we can
    make a similarity argument.
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    And if we want to
    do it formally,
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    we see that both this larger
    right triangle and the smaller
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    right triangle, both have
    a 90-degree angle there.
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    They both have this
    angle in common.
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    So they have two
    angles in common.
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    They are definitely
    similar triangles.
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    And so we know the ratio of
    AH-- let me do it in orange.
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    We know that the ratio
    of AH to AE-- which
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    is 12-- is equal to
    the ratio of AG-- which
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    is 10-- to the ratio of AD--
    which we already figured out
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    was 15.
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    So one way to think about it
    is AH is going to be 2/3 of 12.
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    Or we can just work
    through the math just using
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    the similar triangles.
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    So this right-hand side
    over here is just 2/3.
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    And so AH-- multiplying
    both sides by 12--
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    is equal to 2/3 times
    12, which is just 8.
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    So AH here is 8.
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    AH is 8.
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    AF is 6.
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    So FH right over here
    is going to be 2.
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    And so now we have
    enough information
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    to figure out the area of FHG.
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    So let me write it over here.
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    It's going to be
    1/2 times the base.
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    I'll just use FH
    as the base here,
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    although I could
    do it either way.
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    Well, I'll use FH as the base.
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    1/2 times 2 times the height--
    times 6-- which is equal to 6.
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    And we are done.
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    And you could keep going.
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    You could figure out the
    length of pretty much
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    all of these segments here
    using some of these techniques
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    or any of these areas.
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    Well, we've actually
    figured out most of them.
Title:
Median Centroid Right Triangle Example
Description:

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Video Language:
English
Team:
Khan Academy
Duration:
09:01

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