f(1°)+f(2°)+.+f(59°)= [f(1°)+f(59°)]+[f(2°)+f(58°)] + ... + [f(29°)+f(31°)] + f(30°)
根据和差化积公式 ,cosα + cosβ = 2cos[(α + β)/2]·cos[(α - β)/2]
且有：cosγ = cos(-γ)
∴[f(1°)+f(59°)] = cos1/cos(29) + cos59/cos(-29) = (cos1 + cos59)/cos29 = 2cos30·cos29/cos29 = 2cos30,同理可得：)]+[f(2°)+f(58°)] = ... = [f(29°)+f(31°)] = 2cos30 ,而f(30°) = cos30 ,
∴所求式 = 29·2cos30°+ cos30°=59·cos30°=(59/2)·√3