已知向量2a+b=(0,-5,10),c=(1,-2,-2),a*c=4,|b|=12，则<b,c>=

(2a+b)*c = (0,-5,10) * (1,-2,-2) 2a*c+b*c = -10 b*c = -18 |c| = (1^2 + (-2)^2 + (-2)^2)^0.5 = 3 cos = b*c/( |b|*|c|) = -18/(12*3) = -1/2; = 120°