About this video
Sal finds the angle of a rotation by tracing a point to its image around the center of rotation. He uses the direction (counterclockwise is positive, clockwise is negative) to rule out options, then compares the turn to a right angle to choose 60 degrees for a triangle and -90 degrees for a quadrilateral.
You'll be able to
- tell whether a rotation is positive (counterclockwise) or negative (clockwise).
- estimate the angle of a rotation by comparing it to a right angle.
- trace a point to its image around the center of rotation to find the angle.
Key words
- rotation
- A transformation that turns every point of a shape the same angle around a fixed point.
- center of rotation
- The fixed point a shape turns around, like the origin or point Q.
- image
- The new shape (or point) you get after a transformation, often labelled with a prime, like A-prime.
- angle of rotation
- How far the shape turns; positive means counterclockwise and negative means clockwise.
Transcript
- [Instructor] We're told that triangle A-prime, B-prime, C-prime, so that's this red triangle over here, is the image of triangle ABC, so that's this blue triangle here, under rotation about the origin, so we're rotating about the origin here. Determine the angle of rotation.
So like always, pause this video, see if you can figure it out. So I'm just gonna think about how did each of these points have to be rotated to go from A to A-prime, or B to B-prime, or from C to C-prime? So let's just start with A. So this is where A starts.
Remember we're rotating about the origin. That's why I'm drawing this line from the origin to A. And where does it get rotated to? Well, it gets rotated to right over here. So the rotation is going in the counterclockwise direction, so it's going to have a positive angle.
So we can rule out these two right over here. And the key question is, is this 30 degrees or 60 degrees? And there's a bunch of ways that you could think about it. One, 60 degrees would be 2/3 of a right angle, while 30 degrees would be 1/3 of a right angle. A right angle would look something like this.
So this looks much more like 2/3 of a right angle, so I'll go with 60 degrees. Another way to think about is that 60 degrees is 1/3 of 180 degrees, which this also looks like right over here. And if you do that with any of the points, you would see a similar thing.
So just looking at A to A-prime makes me feel good that this was a 60-degree rotation. Let's do another example. So we are told quadrilateral A-prime, B-prime, C-prime, D-prime, in red here, is the image of quadrilateral ABCD, in blue here, under rotation about point Q.
Determine the angle of rotation. So once again, pause this video, and see if you can figure it out. Well, I'm gonna tackle this the same way. I don't have a coordinate plane here, but it's the same notion. I can take some initial point and then look at its image and think about, well, how much did I have to rotate it?
I could do B to B-prime, although this might be a little bit too close. So I'm going from B to, let me do a new color here, just 'cause this color is too close to, I'll use black, so we're going from B to B-prime right over here. We are going clockwise, so it's going to be a negative rotation.
So we can rule that and that out. And it looks like a right angle. This looks like a right angle, so I feel good about picking negative 90 degrees. We could try another point and feel good that that also meets that negative 90 degrees. Let's say D to D-prime. So this is where D is initially.
This is where D is, and this is where D-prime is. And once again, we are moving clockwise, so it's a negative rotation. And this looks like a right angle, definitely more like a right angle than a 60-degree angle. And so this would be negative 90 degrees, definitely feel good about that.
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