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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.