Measure the longest line. Use a ruler.
We know that triangles are made up of three straight lines or three sides. We’re given a triangle here. And we’re asked to measure the longest of those three sides, the longest line. Which lines are we supposed to measure? We can see which side is the longest by comparing them. We’ll call this line side 𝐴. Is side 𝐴 the longest? What about side 𝐵? Well, side 𝐵 looks like it’s slightly longer than side 𝐴. So it’s the longest line so far. But when you compare them both with side 𝐶, which is this line here, we can see that this is the line that’s the longest.
We need to measure from one end of the line to the other. And we’re asked to use a ruler. The point of this video is going to show how we can use a ruler to measure that longest line. The triangle could look different sizes, depending on the way that you’re watching this video. If you’re watching it on a mobile phone, it could be small or perhaps you’re watching it on an interactive whiteboard when it’s very large. So we can’t actually measure the triangle in real life. Hopefully, if yourself or your teacher has printed out the question, it will be the exact size. This video is just going to show us how to measure correctly and importantly what mistake we need to avoid.
So let’s imagine now we’ve got our ruler out. And we’re going to measure this longest line. Where should we put the ruler? So shall we line our ruler up here? We’ve made sure that the end of the ruler is level with the end of the line. And we can read the measurement from the other end of the line. It looks like it’s halfway between eight and nine centimetres. It must be eight and half centimetres.
Have we measured correctly? No. Look at where the zero is on the ruler. Zero shows the start of the scale on the ruler. This is where we need to start measuring from, not the end of the ruler. Let’s shift our ruler up a little bit so that this time we line up the zero with the start of the line. This time, we’ve managed to line up the zero with the start of the line. So now, we’re counting from zero. And if we read the measurement from the other end of the line, we can see that the line is nine centimetres long.
So although the actual size of the triangle on your screen may be different, depending on how you’re watching this video, we’ve just shown how to line up the ruler and to measure to find the longest line. We need to make sure that our ruler is lined up alongside the line and most importantly that we start measuring from the zero. So we need to match up the zero with one end of the line. If you have time, why not you use a ruler to measure the other two sides just to check that we had chosen the longest side correctly? The length of the longest line of this triangle is nine centimetres.