已知a，b是两个单位向量若丨3a-2b丨=3求丨3a+b丨的值 若丨a—2丨十丨3—b丨=0,试求3a—b的值。
\u5df2\u77e5a,b\u662f\u4e24\u4e2a\u5355\u4f4d\u5411\u91cf\u82e5|3a-2b|=3,\u8bd5\u6c42ab\u7684\u503c\u7b54\u6848\u662f\uff1a3
a,b的夹角=x|3a-2b|=3
9|a|^2+4|b|^2-12|a||b|cosx =9
13-12cosx =9
cosx = 1/3
|3a+b|^2 =9|a|^2+|b|^2+6|a||b|cosx
=9+1+2
=12
|3a+b| =√12 =2√3
答案是14。
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