设向量ab满足lal=lbl=1l3a-2bl=3求l3a=bl=?

求|3a+b|吧?
|3a-2b|²
=（3a-2b）²
=9a²-12ab+4b²
=9|a|²-12ab+4|b|²=3
于是9-12ab+4=3
得ab=5/6
|3a+b|²
=9a²+6ab+b²
=9|a|²+6ab+|b|²
=9+5+1=15
于是|3a+b|=√15