向量b·(2a+b)=b·2a+b·b
=2|a|*|b|*cos120°+|b|^2
=2*4*4*(-1/2)+4^2
=-16+16
=0.
|a+b|^2=(a+b)^2=|a|^2+2a·b+|b|^2
=4^2+2*4*4*cos120°+4^2
=16+32*(-1/2)+16
=16,
∴|a+b|=4,
|2a-b|^2=4|a|^2-4a·b+|b|^2
=4*4^2-4|a|*|b|*cos120°+4^2
=64-4*4*4*(-1/2)+16
=80+32
=112.
∴|2a-b|=4√7.