cos平方20°+ cos平方40°+cos平方80°
=（1+cos40)/2+(1+cos80)/2+(1+cos160)/2
=3/2+1/2(cos40+cos80)-cos20/2
=3/2+1/2*2cos(40+80)/2cos(40-80)/2-1/2cos20
=3/2+1/2cos20-1/2cos20
=3/2