摘要:10.已知|a|=1.|b|=.a与b的夹角为θ. (1)若a∥b.求a·b, (2)若a-b与a垂直.求θ. [解析] (1)∵a∥b.∴θ=0或π. ∴a·b=|a||b|cos θ=1××cos θ=±. (2)∵(a-b)⊥a.∴a·(a-b)=0. 即a2-a·b=0.∴1-1×cos θ=0.∴cos θ=. ∵θ∈[0.π].∴θ=.
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