Question 381187
from f(x)=5x/(x+7), we shall have x<>-7
 
let write f(x)=y, then y*(x+7)=5x=yx+7y
 
x(5-y)=7y => y = 7y/(5-y), hence y should be different from 5
 
Check : x(f(x))=7*5x/(x+7)/(5-5x/(x+7))=35x/(x+7)*(x+7)/(5x+35-5x)
 
=35x/35=x