∵向量a=(1,2,3)b=(-2,-4,-6),∴b=-2a,向量a,b方向相反 a+b=-a,∵(a+b)·c=7∴-a·c=7, a·c=-7又|a|=√(1+4+9)=√14 |c|=√14∴cos=a·c/(|a||c|)=-7/14=-1/2∵两向量夹角∈[0，π]∴ =2π/3
