1. The problem statement, all variables and given/known data [tex]g[/tex] is a continuous and differentiable function on[tex](a,b)[/tex]. Prove that if for all [tex]x\in(a,b)[/tex], [tex]g'(x)\neq0\Rightarrow[/tex] g is an injective function. Is there anything wrong about my proof? I'm not comfortable in making logical mathematical arguments: I usually make small inaccuracies. Or do a lot of work going down a bad path Any help appreciated. 3. The attempt at a solution We select an arbitrary sub-interval on [tex](a,b)[/tex]. We call it [tex](m,n)[/tex] We know that g is also continuous and differentiable on any sub-interval of the interval that it is continuous and differentiable on. The "injective functoin" has the definition that [tex]f(a)=f(b)\Rightarrow a=b[/tex] That is, no two values on the domain can map to the same point on the co-domain. So for now we shall assume that[tex]f(m)=f(n)[/tex]. This allows us to make use of the Mean value theorem which states that if a function is continuous and, differentiable on an interval, in this case [tex](m,n)[/tex] and [tex]f(m)=f(n)[/tex] then [tex]\exists c\in (m,n)[/tex] such that [tex]g'(c) = 0[/tex]. This leads to a contradiction as [tex]g'(x)\neq0[/tex]. Therefor no such interval [tex](m,n)[/tex] can exist. Hence [tex]f(m)=f(n)\Rightarrow m=n[/tex] (The point m on the domain is equivalent to(the same point as) n) and [tex]g[/tex] is injective.