Teorema de Coseno | Introducción

Hey friends! I hope you are doing well. With this video we begin the course on the law of cosines. And in this video we're going to see the most important thing, which is when it applies, how to know in which exercises it does apply and in which exercises it doesn't. And what does

the law of cosines say? Ready? The first and most important thing to understand is that the law of cosines is generally used in oblique triangles. What do you mean by oblique triangles? They are simply triangles that are not right-angled. Yeah? Because? Because if we had an exercise with a right triangle, it's much easier to

solve or find the sides or angles using the Pythagorean theorem, which is much simpler, or trigonometric ratios. Ready? When it is not a right triangle, but an oblique triangle like these types, then the law of cosines applies. Ready? An important point is that it is not mandatory, but generally remember that to make it easier for us,

angles are given the names A, B and C ( uppercase) and sides are given the names A and C (lowercase). I'll do it quickly because we already saw this in the law of sines, right? If we have a triangle, the angles are usually called A, B, and C in uppercase. For example, what would the A be here? Whoever

I want. For example, here this would be called angle A. This would be called angle B and this would be called angle C. You can label any angle you want A, B and C. For example, you can write A, B, and C, no problem. Ready? Now, what's the other important thing? What is important is that the sides will

also be called A, B and C, but in lowercase. Which sides? The opposite sides. In other words, we have angle A, opposite side A. We have angle B, opposite side B, and we have angle C opposite side C. The opposite side would be this one, which we would call C. Why is this?