>I think speed and certain types of memorization are still important. For >example if a student learns sin(2x) = 2sin(x)cos(x), he may forget it 6 >months later but 2 years later he will see sin(2x) and at least know it can >be expanded. The student who doesn't learn these IDs OFTEN won't even know >enough to look it up in a book. >
Can anyone give me a real world *application* that might involve knowing the aforementioned trig identity? I'm not implying that it's not worth knowing. I'm really just curious because, in the 10 years or so since I first learned it, I don't recall ever using it outside of my trig and calculus classes. How do people use these trig identities?