|a-4|+根号b-9=0
a-4=0,b-9=0
a=4,b=9
原式=b^2/(a+b)÷ab^2/(a-b)*(a+b)
=b^2/(a+b)*(a-b)*(a+b)/ab^2
=(a-b)/a
=(4-9)/4
=-5/4