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# Determining rotations (old)

An older where Sal determines the rotation that takes one shape to another. Created by Sal Khan.

Video transcript

Use one rotation to
map quadrilateral ABCD to the other quadrilateral. So to map this one to
this one right over here. Use a number between 0 and 360
degrees to describe the angle. Counterclockwise is positive. So we're going to
want to move it counterclockwise to try
to get it to map there. And the only option
they give us-- because they want us to
do it with one rotation-- is the Rotation tool. And so we have to
think about what do we want to rotate
around, what point? And if we put it
right over here, it looks like this
point, point A, does correspond to this
point right over here. So if we were to rotate
this around not 90, but it looks like 180
degrees around this point, point A would show up over here. It feels like point--
let's see, is that right? Is that right or-- well let's
actually just try it out. Point A would show up over--
no, no, no, that's not right. That doesn't seem
to-- let's try it out. Because if we rotated
180 degrees, oh actually, I was right. It did match up. That's why this is interesting. It tests your
visualization skills. So it did actually match up. And what I did is I put that
point of rotation exactly between those points because
it looked like 180 degrees around this point. So rotation by 180 degrees
about negative 1, negative 1. So the center of rotation
is negative 1, negative 1. The angle of rotation
is 180 degrees. Point A maps to this
point right over here. So point A maps to the
point 1, negative 1. And point C, which is
diagonally opposite point A, maps to this point right over
here, which is 6, negative 6. And we got it right.