tan(arctanx1+arctanx2) =(x1+x2)/(1-x1x2) =sin(π/5)/[1-cos(π/5)] =sin(π/5)/[2sin^2(π/10)] =2sin(π/10)cos(π/10)/[2sin^2(π/10)] =cot(π/10) arctanx1+arctanx2=π/2-π/10=2π/5.
若x1,x2是方程x^2-xsinπ/5+cosπ/5=0的两个根,求证arctanx1+arctanx2=2π/5
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