已知: | a |= 2 ,| b |=3, a 和 b 的夹角为45°,求: (1)当向量 a +λ b 与λ a + b 的夹角为钝角时,λ的取值范围; (2)当λ=-2时,向量 a +λ b 与λ a + b 的夹角的余弦值.
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