 Last video we discovered that we can find missing sides or missing angles for any type of triangle using the sign rule. It doesn't have to be a right angle triangle like Pythagoras and Socrates. But the sign rule doesn't always work. We need a plan B and that is the cosign rule. We will look at how to use the cosign rule in the next video but for now we're going to look at when to use the sign rule and when to use the cosign rule. Remember in the sign rule video I said that you need to label the sides of little a b's and c's and angles with capital a b's and c's and the side a and angle a are always opposite. Side b and angle b are always opposite and side c and angle c are always opposite. This is really important for both the sign and the cosign rule. Right so when does the sign rule work? The sign rule works on pairs. You need one complete pair of a side and this angle and then you need half of the information of the other pair. So this is the complete pair and this is the half pair. We have the 80 degrees and we want the side that's opposite. Can we use the sign rule to find this missing angle? Pause the video, work out the answer and click play when you're ready to check. Yes we can! The 5.1 and 100 degrees are the complete pair and they're opposite and the 3.6 and the missing angle b are the half pair. What about this triangle? Can we use the sign rule to find the missing side? Pause the video, work out the answer and click play when you're ready to check. No we can't! We don't have any complete pairs of opposite side and angle and the half pair. So this means we use the cosign rule. The cosign rule works when we either have all three sides and we want an angle or we have side, angle, side next to each other. Think of it as cosy cosine hugs the angle, side, angle, side. So from this video you should know that if the triangle isn't a right angle triangle we can use either the sign or the cosign rule to find the missing side's angles. You should know that the sign rule needs one complete pair and half of the second pair whereas the cosign rule can either have all three sides given or side, angle, side.