设=α,tanα=2, cosα=1/√5,C=(1,-2),
d=m(-2,5)+n(4,0)=(-2m+4n,5m)
cosα=c*d/|c|*| d|, (1/√5)^2=(-2m+4n-10m)^2/|c|^2*|d|^2
|c|^2=5,
|d|^2=(-2m+4n)^2+(5m)^2=29m^2+16n^2-16mn
1/5=(144m^2+16n^2-96mn)/5(29m^2+16n^2-16mn)
144m^2+16n^2-96mn=29m^2+16n^2-16mn
115m^2=80mn
m/n=16/23