Proving $f(-k)<-k$ when $f(k)<k$

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In summary, to prove the statement that $f(-k)<-k$ when $f(k)<k$, we can use mathematical induction by showing it holds true for a specific value of $k$ and then proving it for all values of $k$. A specific example of this statement is demonstrated using the function $f(x) = x^2 + 1$. It is not necessary for $f(k)<k$ to be true for all values of $k$ in order to prove this statement. Other methods such as contradiction or direct proof can also be used, but mathematical induction is often the most efficient. This statement has practical applications in various areas of mathematics, computer science, and engineering.
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Let $f$ be a polynomial with integer coefficients such that $f(-k)<f(k)<k$ for some integer $k$. Prove that $f(-k)<-k$.
 
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Solution of other:
As $a^n-b^n=(a-b)(a^{n-1}+a^{n-2}b+\cdots+b^{n-1})$, then for any distinct integers $a$ and $b$ and for any polynomial $f(x)$ with integer coefficients $f(a)-f(b)$ is divisible by $a-b$.

Thus, $f(k)-f(-k)\ne 0$ is divisible by $2k$ and consequently $f(-k)\le f(k)-2k<k-2k=-k$.
 

FAQ: Proving $f(-k)<-k$ when $f(k)<k$

How can I prove that $f(-k)<-k$ when $f(k)

To prove this statement, we can use the concept of mathematical induction. First, we can show that the statement holds true for a specific value of $k$, such as $k=1$. Then, we can assume that the statement holds true for some arbitrary value of $k$, and use this assumption to prove that it also holds true for $k+1$. This will establish that the statement holds true for all values of $k$.

Can you provide a specific example to illustrate this statement?

Sure, let's consider the function $f(x) = x^2 + 1$. If we plug in $k=1$, we get $f(1) = 1^2 + 1 = 2$. This means that $f(k) < k$ for $k=1$. Now, let's assume that $f(k) < k$, and let's see if we can prove that $f(k+1) < k+1$. We have $f(k+1) = (k+1)^2 + 1 = k^2 + 2k + 2$. Since $f(k) < k$, we can substitute $f(k)$ with $k$ to get $k < k+1$. Therefore, we can conclude that $f(k+1) < k+1$, and the statement holds true for all values of $k$.

Is it necessary for $f(k)

No, it only needs to be true for the specific value of $k$ that we are using to prove the statement. As shown in the previous example, we only needed $f(k)

Can this statement be proven using other methods besides mathematical induction?

Yes, there are other methods that can be used to prove this statement, such as contradiction or direct proof. However, mathematical induction is often the most efficient and straightforward method for proving statements of this form.

What are the practical applications of proving $f(-k)<-k$ when $f(k)

This statement is commonly used in various mathematical proofs and can be applied to many different areas of mathematics, such as algebra, calculus, and number theory. It can also be useful in computer science and engineering, where functions and inequalities are often used to model real-world problems.

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