Exact value of cos20

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2011-12-08 21:49:02 補充：
u is the cube root of a complex number, there are 3 possibilities
v=1/4u is also a complex number and indeed the complex conjugate of u
The imaginary part of u and v will cancel out leaving only real numbers

2011-12-08 21:50:53 補充：
The 3 sets of real values correspond to sin20, sin40 and sin -80. You have to determine which one is sin20 by simple judgement.

cos(20°)=[(1+i√3)/16]^(1/3) +[(1-i√3)/16]^(1/3)

cos(20°) ?