∵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