cos（α+β）=cosαcosβ-sinαsinβ cos75°=Cos(30°+45°) =cos30°cos45°-sin30°sin45° =√3/2×√2/2-1/2×√2/2 =√2/4 ×(√3-1) =1.414/4 ×(1.732-1) =0.25876
