Let, x be any poistive real number.
⇒x<5x<7x
⇒f(x)<f(5x)<f(7x)
⇒f(x)f(x)<f(x)f(5x)<f(x)f(7x)
⇒1<f(x)f(5x)<f(x)f(7x)
⇒x→∞lim1<x→∞limf(x)f(5x)<x→∞limf(x)f(7x)
It is given that, x→∞limf(x)f(7x)=1
So, by using Sandwich theorem,
⇒x→∞limf(x)f(5x)=1
⇒x→∞limf(x)f(5x)−1=0