已知向量a+向量b+向量c=0 a的模为3 b的模为5 c的模为7 a与b夹角为60 问是否有d使da+b与a-2b共线

a+b+c =0 |a|=3,|b|=5,|c|=7 a与b夹角=π/3 da+b与a-2b共线 => da+b = k(a-2b) ( k is a constant) => d = k and -2k=1 => d = -1/2