已知向量a和b的夹角为60度，且|a|=4,|b|=5,则（2a-b)?b=几

a?b = |a|?|b|cos60° = 4×5×(1/2)=10; (2a-b)?b = 2a?b - |b|^2 = 2×10 - 25 = - 5;