(ax+3)(bx+5)=k
ax+3=kbx+5k
(a-kb)x+(3-5k)=0无论x取何值均成立
所以
a-kb=0,3-5k=0
k=3/5,
a=kb=3b/5
(a+b)b=(3b/5+b)/b=(8b/5)/b=8/5