This is a relatively old video, but I have an IGCSE mathematics extended exam tomorrow, and wasn't able to understand this section of trigonometry for the life of me. Thank you so so much for your help!!!
Its part of the advanced math curriculum for year 10 in New South Wales Australia. Known locally as maths 5.3. Every school program varies by state or country which might by why your year 10 program might not include this topic.
To find a reference angle you always want to find the acute angle to the horizontal line. For example in the case of sin240. 240 falls in the third quadrant, if you subtract 240 - 180 you get the 60 degrees which you can then find the exact value of from the exact value triangles. I hope this answers your question... unless your question was about the exact value of sin240 which starts by taking 240 -180 = 60? If you could clarify your question ill try to give a better answer. All the best.
