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Determining rotations (old)

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

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Video transcript

- [Instructor] 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 zero and 360 degrees to describe the angle. Counter-clockwise is positive. So we're going to want to move is counter-clockwise to try and get it to map there. And the only option they give us 'cause they want us to do it with one rotation is the rotation tool. And so we have to think about what do you 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, 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 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. 'Cause 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 but because it looked like 180 degrees around this point, so rotation by 180 degrees about negative one negative one so the center of rotation is negative one negative one, the angle of rotation is 180 degrees. Point A maps to this point right over here, so point A maps to the point one negative one, one negative one. And point C point C which is diagonally opposite point A maps to this point right over here, which is six negative six, six negative six, six negative six and we got it right.