【优化指导】2015年高中数学3.1.1两角差的余弦公式学业达标测试新人教A版必修41.下列式子中,正确的个数为()①cos(α-β)=cosα-cosβ;②cos=sinα;③cos(α-β)=cosαcosβ-sinαsinβ.A.0B.1C.2D.3解析:三个式子均不正确.答案:A2.若a=(cos60°,sin60°),b=(cos15°,sin15°),则a·b=()A.B.C.D.-解析:a·b=cos60°cos15°+sin60°sin15°=cos(60°-15°)=cos45°=.答案:A3.cos75°cos15°-sin255°sin15°的值是()A.0B.C.D.-解析:原式=cos75°cos15°+sin75°sin15°=cos(75°-15°)=cos60°=.故选B.答案:B4.设α∈,若sinα=,则cos=______.解析:∵α∈,sinα=,∴cosα=,∴cos==+=.答案:5.化简求值:cos(x+27°)cos(x-18°)-cos(63°-x)sin(18°-x).解:原式=cos(x+27°)cos(x-18°)+sin[90°-(63°-x)]·sin(x-18°)=cos(x+27°)cos(x-18°)+sin(x+27°)sin(x-18°)=cos[(x+27°)-(x-18°)]=cos45°=.1