y=(x²-1+12)/(x+1)
=[(x+1)(x-1)+12]/(x+1)
=(x+1)(x-1)/(x+1)+12/(x+1)
=x-1+12/(x+1)
则12/(x+1)是整数
所以x+1是12的约数
因为x为自然数
所以x+1=1,2,3,4,6,12
所以x=0,1,2,3,5,11
所以和=0+1+2+3+5+11=22
(2)设(ax+3)/(bx+5)=k,k为定值
ax+3=kbx+5k
(a-kb)x=5k-3
当a-kb=0,5k-3=0
即k=3/5,a=3b/5时,满足题意
(a+b)/b=8/5