已知i,j,k是空间中两两垂直的单位向量,所以
i^2=j^2=k^2=1,ij=jk=ik=0
cos
=(i+j)(j-k)
=ij-ik+j^2-jk
=1
｜m｜=｜i+j｜=√(i+j)^2=√2
｜n｜=｜j-k｜=√(j-k)^2=√2
cosa=(m*n)/(｜m｜*｜n｜)=1/2
=60°