A short instructional video (presenter Ben) teaching primary-school students how to calculate missing angles: on a straight line (180°), within a full turn/complete circle (360°) including reflex angles, and vertically opposite angles. The presenter emphasizes using visual aids (Anne the angle eater, an angle turner), annotation, and step-by-step arithmetic strategies (e.g., 180−150=30; 360−(90+60)=210).
Chapters
Hi, my name's Ben. In this video, we're going to be looking at how to calculate missing angles. Now, in a previous video, we've discussed these. This is Anne, the angle eater, and an angle turner. And these are really useful for getting children to have a good understanding of what angles are and their angle names before we even progress into calculating angles on a missing, on a straight line or in a reflex angle. And really important that children understand those key facts. Okay, here, this is calculating angles on a straight line. The key fact here is children have to know that a straight line equals 180 degrees. And then they can work out what the missing part is, because they can realize that 180 minus the 150 degrees, 30 degrees, or what do I have to add to 150 to get to 180? So 180 minus 150 equals 30 degrees. Okay, this next question is a lot more tricky and there's a lot of work that the children have to do here. For me, when I teach this, I teach that annotation is key. So there's a lot going on. First of all, they have to realize that this symbol here denotes a right angle, which is 90 degrees. And they have to realise that they're dealing with a whole turn here, which is 360 degrees. And then in order for them to calculate what this missing angle here is, they'll have to do 90 plus 60, which equals 150, and then subtract that from the full turn. So 360 minus 150 degrees. What's really useful, if the children have got that good understanding of the size of angles, when they do 360 minus 150 to get 210, which is this angle here, they will realize that that's a reflex angle and that fits. And that's where that good understanding of angle names comes in. The final one is this one here. And again, the crucial piece of information is that children have to understand that opposite angles, when equal, always equal the same measurement. So here, if angle A equals 50 degrees, angle B, which is this one, will also equal 50 degrees. And here another useful tip is to get a piece of paper and put it over. And the children can then see that that is a straight line and therefore 180 degrees. So if that is 50, this angle here will be 130. If D is 130 degrees, so will C be 130 degrees. And the final check is that they could total all those numbers and then we'll get 360 degrees, which is a complete circle. In order for children to access those tricky problems, they have to have that real good grounding of what angles are.
