m⊥n
(a+b)(a+λb)=0
a^2+λab+ab+λb^2=0
a^2+(λ+1)ab+λb^2=0
|a|^2+(λ+1)|a||b|cos135+λ|b|^2=0
18-14(λ+1)+16λ=0
18-14λ-14+16λ=0
2λ+4=0
λ=-2