y1与x+2成正比例,y2与x成反比例
则可设y1=k(x+2),y2=t/x
则y=y1-y2=k(x+2)-t/x
当x=1时,y=0,当x=4时,y等于7.
则0=3k-t
7=6k-t/4
两式联立,解得k=4/3,t=4
则y=4/3(x+2)-4/x
x=2时,y=4/3*4-4/4=13/3