How do you find the exact value of #cos68# using the sum and difference, double angle or half angle formulas?
1 Answer
Jun 6, 2018
This angle is not constructable, so its cosine will not be related to any sum and difference formulas made out of all constructible angles . This means there won't be any closed formed expression combining integers using addition, subtraction, multiplication, division and square roots. There is a way to get something using nth roots of complex numbers but that's not really the sum and difference formulas so I'll skip that here.
Just to pedantically answer the question,
That's exact, though with trig functions of