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【简答题】
已知向量 a ， b ，满足 a =(1 ， 2) ， b =(-2 ， 1) ． （1）求向量 a - b 的坐标，以及向量 a - b 与 a 的夹角； （2）若向量 a - b 与 k a + b 垂直，求实数k的值．
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