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此題可以用投影法來計算.設正方形邊長為1,所求夾角為θ.意得⊿PCD在平面PAB的投影為⊿PAB. cosθ=S⊿PAB÷S⊿PCD(⊿PCD為直角三角形,PC,CD為直角邊) =(1×1/2)÷(1×√2/2) =√2/2 故θ=45°
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