λa+μb=(3λ+5λ-2λ)+(2μ+μ+4μ)=(3λ+2μ,5λ+μ,4μ-2λ)
z=(0,0,n)
垂直,所以 z(λa+μb)=(3λ+2μ,5λ+μ,4μ-2λ)(0,0,n)=0
(4μ-2λ)n=0
解得 2u= λ