4a-2b=(-2,2√3),c=(1,√3),则有
(4a-2b)*c=(-2*1+2√3*√3)=4,
4ac-2bc=4,而ac=3,
4*3-2bc=4,
bc=4.
|b|=4.
|c|=√(1+3)=2.
cos(b,c)=bc/|b|*|c|=4/(4*2)=1/2=cos60.
则,b与c的夹角为:60度.