(cos20°)^4+(cos40°)^4+(cos80°)^4=(cos20°)^4+(cos40°)^4+(sin10°)^4=1/4*((cos40°+1)^2+(cos80°+1)^2+(1-cos20°)^2)=1/4*(1+2cos40°+(cos40°)^2+1+2sin10°+(sin10°)^2+1-2cos20°+(cos20°)^2)=1/4*(...