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Recognizing shapes | Geometry | Early Math | Khan Academy

I have a bunch of shapes here, let’s think
about what they are so this first shape here, you probably
recognize it is the shape of the moon in the sky or atleast on a full moon and we call it a circle. Now what makes it a circle when you get
further in math we come up with fancier ways to define a circle but one with
think about it is if we take the center the circle and if we go from one side
of the circle to the other side circle through the center that’s always going to be the same
length so that’s the same length as is this as this is on his way to the
center the circle as that now if that the circle what would something
like this to be what you might call this an oval. all circles are ovals but this
oval is not a circle because if you have its center see right over there this distance going
in that direction is going to be much shorter or just
shorter than this distance right over here so
this one right over here is not a circle, this one is. Now what
about the shape over here that has three sides one two three you could also say it has three corners
one two three if you have three sides we call that a
triangle Tri angle whenever you see this Tri over
hear that’s a good sign that you’re dealing with the number
three. This is a triangle has three sides note about these shapes
over here these shapes are interesting how many
shapes. So how many sides do these shapes have. Well if you count them you see that this one up here in purple has one two three four sides this one has one two three four sides, this has one two three four sides, this has one two three four sides. So they all have four sides. So a name for any shape that has four
sides is quadrilateral Quad dri quadrilateral quadrilateral and when you see this quad
at the beginning of the word it’s a pretty good sign that you’re
dealing with the number four four sides, four sides that’s what quadrilateral means but
these are all different special cases of quadrilaterals and actually you could even have a quadrilateral that
looks like this, could even have a quadrilateral that look like this. One two three four sides as long as as four sides
it’s a quadrilateral Let’s think about each of these so this
shape right over here what you might see is the corners the
corners all come in, sometimes is called square
sometimes you could call it a right angle so and notice I could put little small squares
here. SO if all four corners are square or they’re all come in at a right angle like this then you’re dealing with a
rectangle you are dealing with the rectangle and so
just to be clear this down here is not a rectangle notice
this corner right over if I tried to pull all square here it would either look like this or
doesn’t it doesn’t the two sides aren’t two
sides of a square or would look like or it would look like
this so this is not a square corner neither
it is this or neither is this so this is not a
rectangle all four corners need to be square this
one also is not a rectangle notice the corners are not the corners are not right angles so this is a direct rectangle this is a
direct rectangle and what about this well I can do that I can put little
squares I could put all squares in the corner or I could see that they are all right all
the corners are right angles so this actually is a
rectangle as well so these are both these are both
rectangles now you might recognize this purple one
on the left is a special type a rectangle not only are all the corners square not all do they have these right angles but all the sides are
the same length at least it looks like it from the
picture this side is the same length as this side is the same length as that side same length as that side. So if the sides are
of same length and all the corners are square where they have right angles,
then we call it a square so this right over here is a, this right over here is a square special type
a rectangle this one not all the side to the same
this one all the sides are the same out if we go down here looks like all
the sides are the same one two three four but it doesn’t have square corners. Notice if I tried to put
a little square here it wouldn’t fit perfectly, if I tried
square here doesn’t fit perfectly these aren’t right angles So if all four sides are the same and
that’s all we know about it we call it a rhombus. We call it a
rhombus so you might be saying, the square all
four sides are the same, is that a rhombus? yes it is. So the square is a rectangle it’s a rectangle and is a rhombus so this last shape here
this last quadrilateral what’s interesting about it is you have two of the side’s going in the same
direction so this one right over here knows what I
mean by the same direction it sometimes is called parallel if I draw two lines here these two lines
are never going to cross each other so these two lines are parallel if you
have opposite one pair of opposite sides that are parallel we call them trapezoids we call them
trapezoids. Notice the other blinds the other
opposite sides these will cross, these will cross eventually to that right over there is
called a trapezoid.

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