Suppose , are differentiable and that g(x) and g'(x) are always non-zero. Suppose that and that

{Equation *}

then .

The usual version of L'Hopitals that I know how to prove involves us allowing to say (from continuous limits) that f(b)=g(b)=0 and thus we can then see that

But here, g(x) is non-zero for all x, so I can't really do this. Furthermore, the fact that we're only dealing with a left limit makes things harder for me. I'm not quite sure where to start, I'm sure it's a simple solution but I can't see it.

I know the definition of {Equation *} is

I literally don't even know where to start anymore, I tried going straight from this definition and doing a bit of jiggery pokery but ran into a lot of problems. I've also taken a look at Cauchy's Mean Value Theorem but don't know what I can do with that, I can't say for certain that these functions are continuous on [a,b].

Can anyone help me find the path to a proof? Cheers.