cos(π÷5)+cos(2π÷5)等于多少?不要用计算器!

问题描述:

cos(π÷5)+cos(2π÷5)等于多少?不要用计算器!

cos(π/5)-cos(2π/5)=cos(π/5)+cos(3π/5)=cos(2π/5-π/5)+cos(2π/5+π/5)=cos(2π/5)*cos(π/5)+sin(2π/5)*sin(π/5)+cos(2π/5)*cos(π/5)-sin(2π/5)*sin(π/5)=2cos(2π/5)*cos(π/5)=2sin(π/10)*cos(π/5...