｜a｜=｜b｜=1,则a^2 =1,b^2=1.
｜3a-2b｜=3,平方得：
9a^2-12a•b+4b^2=9,
9-12a•b+4=9,
a•b=1/3.
｜3a+b｜^2=9a^2+6a•b+b^2=9+2+1=12,
｜3a+b｜=2√3.