Trigonometry: Right angled trigonometry What is a right-angled triangle? Well basically you should know that it isa triangle with one angle of 90° like the one in Fig.1! Now remember thatthe 3 angles of a triangle add up to 180°! Also note that in Fig.2 theisosceles triangle is labeled with alphabets! This is the notation that youneed to know because many of the coming theorems that you will learnwill be expressed in this way! The capital alphabets represent the angleson the triangle and the lower case alphabets represent the length of theside! (Notice that the same letter is given to every angle and its oppositeside!) You should already know that: Sin θ = Opposite/Hypotenuse Cos θ = Adjacent/Hypotenuse Tan θ = Opposite/Adjacent A quick way to memorize this is SOH-CAH-TOA. If you don’t know italready, it’s the first letters of each rule!