Question

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

Answer 1

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