Count the acute angles in this picture.
Well what do we mean when we say acute angle? Well we mean an angle which is between zero and 90 degrees but not including zero and 90 degrees. So if it’s zero degrees, it doesn’t count as an acute angle. If it’s 90 degrees, it doesn’t count as an acute angle. But anything in between is acute.
Now all of these angles are 90 degrees, and we call them right angles, so they’re not acute. And these angles are all greater than 90 degrees. So if they’re between 90 and a 180, not including either of those two, then they’re called obtuse. And if they’re bigger than a 180 but less than 360 still, then they’re called reflex; they’re not acute angles, and that doesn’t leave us with many angles.
And in fact, I’ve spotted two more right angles, this one and this one. I missed those earlier when I was counting them, so which ones have we got left? This one is between zero and 90 and so is this one. But look, that angle there is also between zero and 90 and so is that one.
Well I think we’ve classified every single angle we can find in that diagram now, and four of the angles were acute.