向量 l 1 与 l 2 满足 | l 1 |=2,| l 2 |=1 ,且夹角为60°, f(x)=(2x? l 1 +7? l 2 )?( l 1 +x? l 2 ) ,(x∈R). (1)求函数f(x)的解析式. (2)当f(x)=-15且2x+11≠0时,求向量 2x? l 1 +7? l 2 与向量 l 1 +x? l 2 的夹角.
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