p点代入y=2/x得:a=2/2=1
p点代入y=mx得:a=2m=1,即m=1/2
q点代入y=mx得:4=bm=b/2,即b=8
q点代入y=n/x得:4=n/b=n/8,即n=32
p,q的距离为√[(2-8)^2+(1-4)^2]=3√5