I will use d for degrees.
cos80dcos20d - sin80dsin20d =
I got -0.173648178
Is this correct?
Hello magentaritaIf and , then the formula becomes
So to work out the value of you can simply use a calculator and find instead.
Or, of course, you can do it the long way and find the separate values of and , and then multiply and add. You should get the same answer whichever way you choose.
Grandad
Hello magentaritaI'm not quite sure what you mean by this. Obviously you don't mean that you can't find two numbers that add together to make 100.
Do you mean how do you find without using a calculator? If so, I'm not sure that you can. There are some angles - like and so on - that have easy sines and cosines, and you may not need a calculator for these. And you may be able to find two of these special angles that will combine to give (perhaps by adding, subtracting, doubling, and so on), but I can't think of any at present.
Is this what you meant?
Grandad