向量的坐标运算已知m=(1,0),n=(1/2,(√3)/2 ),a=m+2n,b=2m-3n.,则a与b的夹角为

a = m + 2n
= (1, 0) + 2(1/2, √3/2)
= (2, √3)
b = 2m - 3n
= 2(1, 0) - 3(1/2, √3/2)
= (1/2, -3√3/2)
cos = (a • b)/(∣a∣∣b∣)
= [2*(1/2) + √3*(-3√3/2)] / { [√(2²+√3²) ] [√( (1/2)²+(-3√3/2)² )] }
= (-7/2) / 7
= -1/2
∴a, b的夹角为120度.