题目
计算cos(+α)+cos(+α)+cosα. 答案:分析:构造三个向量a=(sinα,cosα),b=(sin(π+α),cos(π+α)),c=(sin(π+α),cos(π+α)),利用a,b,c之间的关系求解. 解:令a=(sinα,cosα),b=(sin(π+α),cos(π+α)),c=(sin(π+α),cos(π+α)).如图所示,|a|=|b|=|c|=1,cos∠AOB==sinαsin(π+α)+cosαcos(π+α)=cosπ,同理,cos∠BOC=cos∠COA=cosπ.由向量加法的几何意义得a+b=-c.所以a+b+c=0,即cosα+cos(+α)+cos(+α)=0.下列腺体属于内分泌腺的是
①皮脂腺②唾液腺③胸腺④肝脏⑤胰岛⑥汗腺⑦垂体⑧肠腺⑨肾上腺⑩卵巢胃腺甲状腺泪腺睾丸
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A.①③⑤⑦⑨B.②④⑥⑧⑩
C.①②④⑥⑧D.③⑤⑦⑨⑩