(1)因为y1与x+1成正比例,y2与x+1成反比例
设y1=k1(x+1),y2=k2/x+1
y=y1+y2,则y=k1(x+1)+k2/x+1
当x=0时,y= -5;当x=2时,y= -7
-5=k1(0+1)+k2/(0+1)
-7=k1(2+1)+k2/(2+1)
k1=-2,k2=-3
y=-2(x+1)-3/(x+1)
(2)当x=5时
y=-2*6-1/2=-12.5