Let $f:\mathbb{R}\rightarrow\mathbb{R}$ be a function satisfying the conditions
$$\begin{align*} (1)&f\left(\frac{x+y}{2}\right)=\frac{f(x)+f(y)}{2}\\ (2)&f(0)=1\\ (3)&f'(0)=-1 \end{align*}$$ Find the value of $f(2)$ by proper explanation.
Find the value of the function at the given point.
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0You can get appropriately sized parentheses (and other paired delimiters like brackets and braces) by placing `\left` and `\right` in front of them. – 2012-12-28
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0@JonasMeyer: Can we say that the function is not concave upward cause of 1?? – 2012-12-28
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0I have solved it by thinking that it is neither concave or convex function but it is a straight line. – 2012-12-28
3 Answers
To begin with, let us introduce the function $g(x)=f(x)+x-1$. This function satisfies the midpoint property (1), and $g(0)=g'(0)=0$. We claim that these three conditions imply that $g(x)=0$ for all $x\in\mathbb{R}$.
Since $g(0)=0$, it is easily seen that (1) implies $g(-x)=-g(x)$ for all $x$.
Next observe that $g(x)-g(y)=2g((x-y)/2)$ for all $x,y$. It follows that $g$ is differentiable on $\mathbb{R}$ and that $g'(x)=g'(0)=0$ for all $x$.
Hence $g$ is constant and equal to $g(0)=0$. So $f(x)=1-x$ for all $x$.
In particular, we find $f(2)=-1$.
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0We do not get that it's differentiable unless $f$ is continuous on a neighborhood of zero. – 2012-12-28
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0@mixedmath From the condition $f'(0)=-1$, I deduce that $f$, hence $g$ is differentiable at $0$. Then from the transformed functional equation $g(x)-g(y)=2g((x-y)/2)$, differentiability at $0$ (RHS), implies differentiability at $x$ (LHS). – 2012-12-28
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0Ah, you're right. We are given that derivative, so one would assume. Well done, +1 – 2012-12-28
First, an example of a function that satisfies all the given conditions: $g(x) = 1 - x$. Clearly, if we trust that the problem has only one answer, the answer is bound to be $g(2)=-1$.
Of course, this won't pass as a complete solution, because now we need to somehow prove that $f(2)=-1$ for an arbitrary $f$ that satisfies the three conditions. There is no guarantee (at least no obvious guarantee) that such an $f$ must be equal to $g$. And here is the main idea: it is quite easy to prove that $f(x)=1-x$ for any dyadic number $x \in \mathbb{R}$. It will automatically mean that $f(2)=-1$.
PS: and if you think just a bit more you can even prove that $f(x)=1-x$ for every $x \in \mathbb{R}$.
PPS: the answer is meant as a hint, but I can expand upon request.
UPDATE: expanding, although now with all these other answers there is little point to it.
The proposed solution would go something like this. Step one: let's explore what (1) tells us. Let's call $\alpha = f(0)$ and $\beta = f(1)-f(0)$. So, we have the equality $f(x) = \alpha + \beta x$ for $x = 0$ and $x = 1$. Using (1) it is almost trivial to expand this equality first to $\mathbb{Z}$, and then to the set of all dyadic numbers. So, for all dyadic numbers $x$ we have $f(x) = \alpha + \beta x$. Using (2) we now get that $\alpha = 1$, and using (3) we get $\beta = -1$.
What I said in PPS is a simple variation of the same logic.
This is the way I was thinking about the problem. If you strip it down to the minimum, you'll get a solution similar to WimC's answer.
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0please expand your answer. – 2012-12-28
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0@rajkamalmath Done. – 2012-12-28
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0hey friend what about thinking the function as straight line, I am telling it by seeing property(1), is it wrong.. – 2012-12-28
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0@rajkamalmath This is the whole point. Property (1) alone is not enough to conclude that the graph of $f$ is a straight line, one can build a counterexample. I don't see an elementary counterexample, but I do see a construction using the basis of $\mathbb{R}$ over $\mathbb{Q}$. – 2012-12-28
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0@rajkamalmath Although if you add to (1) the restriction that $f$ is continuous, then it will automatically mean that $f$ is affine, i.e. its graph is a line. In this problem, continuity of $f$ can be proved, but only because of condition (3). If we only had (1) and (2) without (3), then it would be incorrect to say that $f$ is affine. – 2012-12-28
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0Nite answer +1 $\ddot\smile$ – 2012-12-29
From (1) and (2): $2^k\left(f(2^{1-k})-1\right)=f(2)-1$ for all $k \geq 0$.
From (3): $\lim_{k\to \infty}2^k\left(f(2^{1-k})-1\right)=-2$.
So $f(2)=-1$.