证明：cotθ+cscθ=cosθ/sinθ+1/sinθ
=(cosθ+1)/sinθ
=[2cos(θ/2)]^2/[2sin(θ/2)cos(θ/2)] (应用倍角公式)
=cos(θ/2)/sin(θ/2)
=cot(θ/2)
即 左边=右边,原式成立.