设向量a.b满足|a |=|b |=1,|3a-2b |=3,求|3a+b|
人气：494 ℃ 时间：2020-03-28 09:50:13
解答
|3a-2b |^2=9
9a^2+4b^2-12ab=9
又因为|a |=|b |=1
所以 9a^2+4b^2-12ab=9 可化为
9+4-12ab=9
ab=1/3
|3a+b|^2=9a^2+b^2+6ab=9+1+6*1/3=12
|3a+b|=2根号3
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