用向量的积(?)cos(?),可求向量(?)的夹角,但应用于求角时,必须注意所用向量的方向．否则,极易出错．例1已知△ABC的顶点A(-1,0)、B(1,0)、C(1,2),求∠C的度数．解1 (?)=(2,2),(?)=(0,-2),cosC=(?)=-2~(1/2)/2,故∠C=135°．解2 (?)=(2,2),(?)=(0,2),cosC=(?)=2~(1/2)/2,故∠C=45°．剖析两个解法结果互异,谁对谁错?原因何在?描点作图易知∠B=90°,|AB|= Using the product (?)cos(?) of the vector, the angle of the vector (?) can be found, but when applying to angles, one must pay attention to the direction of the vector used. Otherwise, it is easy to make mistakes. Example 1 knows the vertices A(-1,0), B(1,0), C(1,2) of △ABC, and finds the degree of ∠C. Solution 1 (?)=(2,2),(?)=(0,-2),cosC=(?)=-2~(1/2)/2, so ∠C=135°. Solution 2 (?) = (2,2), (?) = (0,2), cosC = (?) = 2 ~ (1/2)/2, so ∠ C = 45 °. Analyze the results of the two solutions that are different from each other. Who is right or wrong? What are the reasons? Sketching is easy to know B=90°,|AB|=