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Try this beautiful Functional Equation Problem from SMO, Singapore Mathematics Olympiad, 2013.

## Problem – Functional Equation (SMO Test)

Let M be a positive integer .It is known that whenever $|ax^2 + bx +c|\leq 1$ for all

$|x|\leq 1$ then $|2ax + b |\leq M$ for all $|x|\leq 1$. Find the smallest possible value of M.

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### Key Concepts

Functional Equation

Function

But try the problem first…

Source

Challenges and Thrills – Pre – College Mathematics

## Try with Hints

First Hint ………………….

We cant this sum by assuming a,b,c as fixed quantity.

Let $f(x) = ax^2 + bx + c$.

Then $f(-1) = a – b + c$ ; $f(0) = c$ ; $f(1) = a + b + c$ ;

Try to do the rest of the sum …………………………..

Second Hint ……………………..

Suppose $|f(x)|\leq 1$ for all $|x|\leq 1$ . Then

$|2ax + b| = | (x – \frac {1}{2} ) f(-1) – 2 f(0) x + (x+\frac {1}{2} f(1) |$

$\leq |x – \frac {1}{2}| + 2 |x| + |x + \frac {1}{2}|$

$\leq |x – \frac {1}{2} | + |x+\frac {1}{2}| + 2$

$\leq 4$

Final Step ………………………….

Now I guess you have already got the answer but if not ………….

From the last step we can conclude ,

$|2 x^2 – 1|\leq 1$ whenever $|x|\leq 4$ and $|2x| = 4$

is achieved at $x = \pm 1$.