空间向量题 设|m|=1,|n|=2,2m+n与m-3n垂直,a=4m-n,b=7m+2n,

因为 2m+n 与 m-3n 垂直,因此 (2m+n)*(m-3n)=0 ,展开得 2m^2-3n^2-5m*n=0 ,即 2-12-5m*n=0 ,解得 m*n= -2 ,所以,a*b=(4m-n)*(7m+2n)=28m^2-2n^2+m*n=28-8-2=18 ,a^2=(4m-n)^2=16m^2+n^2-8m*n=16+4+16=36 ,b^2=(7m+2n...