Respect for Ivan Sidorov who posted a 20-second video in 2012 to show how this is done. It is *so* simple. Here’s the video and a
Now this looks like a really good idea…. With links to lots of “how to” videos and questions.
What would you draw for two thirds plus one quarter?
This video covers:
There are loads of different ways to approach the problem of “How do I remember that the total of the angles in a hexagon is 720?” (and all the other polygons for that matter!). Which one suits YOU is pretty dependent on your learning style, and on how much Maths you do week on week….
Memorisation (and a cheat!)
This may work for you, to some extent. You need to KNOW that the total of angles in a triangle is 180 degrees. If you really struggle with this, perhaps it can help you to dig out your protractor from your pencil case. The biggest number on it is 180 and that’s the number you need to remember!
Again most people can remember that a RIGHT ANGLE is 90 degrees so the TOTAL of the angles in a rectangle is 360. (4×90).
If you are really good at memorising, then it may be helpful for you to remember that, every time you add one more side to your polygon, you add another 180 degrees. That means that a 5 sided shape (pentagon) has 540, 6 sided has 720, 7 sided has 900, 8 sided has 1080, etc etc…..
One single angle in a regular polygon
For example, an 8 sided regular octagon has a total of 1080 degrees. Each internal angle must be 1080/8 = 135 degrees.
This may suit you if you really have a flaky memory…. it IS important that you know The total of the angles in a triangle is 180 degrees. Then you can sketch the shape…. split it into triangles….. see the video below.
I have taught this the “traditional” way where you break the octagon up into 4 triangles. It’s cool. Mathematically very clever. But VERY hard for some of my students to actually remember… so this (less elegant) way is what I tend to teach more often.
Working Backwards is almost as easy….
Sometimes you are given the interior angle and told “This is from a regular polygon. How many sides has it got? Here’s how to do that kind of question….
Stage 1 – practice squaring with pen and paper, until you are confident (see video below)
Stage 2 – just jot the answers into the grid
Stage 3 – jot the grid and imagine the numbers
Stage 4 – do it all in your head
The video for that would be a bit boring!
D. made this film at the end of yesterday’s tutorial. He chose 4 equations that summed up what he had learned during the hour’s lesson. I am indebted to M.F. who originally noticed the easy trick for plotting lines which have a fractional gradient.
Chinese multiplication has been explained many times in many places on the Internet. This is a quick recap of the way I do it….
The kids I’ve taught, especially the more able ones, really like this way of multiplying numbers because it’s SOOOO easy to build up to very large numbers.
Within 20 minutes, a group of ambitious mathematicians has commandeered the class whiteboard and tried to do an ENORMOUS sum like 185936296722 x 15436796 and got an answer. This gives the teacher a problem. How can the sum be checked? Calculators and EXCEL will round the answer to only 10 or so significant figures, which is pretty hopeless for checking the work.
Here is a link to an EXCEL spreadsheet that will do these HUGE sums so you can check pupils’ (or your own) work.
The extra challenge that these interpid mathematicians give themselves, of course, is how to add together huge long lists of numbers. Here’s an example of one of the additions in the sum mentioned above:
The student has to add 4,1,7,5,2,1,2,5,7,1,4,2,5,1,2 and 0. It’s tough to add all that without errors, so encourage them to look for TENS, and cross them out, “carrying” them into the next column…
They could make ten from the 4,1 and 5, then another from 7,2 and 1, and anotherfrom 1,4 and 5. Cross them out neatly and there’s not really much more to add! The nice thing is you can tackly any column you like, in any order, which is great for mathematicians who don’t know their right from their left! (except of course the TENS have to move left!).
Once this TEN-hunting is complete, the final pass is to add up any digits that are left.
Finally, some thoughts about the process of learning Chinese Multiplication: