1
$\begingroup$

What two nonnegative real numbers $a$ and $b$ whose sum is 23 maximize $a^2+b^2$? Minimize $a^2+b^2$?

3 Answers 3

3

If $a+b = 23$ then $b=a-23$ so just maximize and minimize it $2a^2-46a+23^2$ using calculus.

  • 1
    Should be $-46$2012-10-31
3

Hint:

$2(a^2 + b^2) = (a + b)^2 + (a - b)^2 = 23^2 + (a - b)^2.$

So to maximize (minimize) $a^2 + b^2$, you should maximize (minimize) $(a - b)^2$.

1

You could make use of the following inequality. $(a+b)^2\leq 2(a^2+b^2)$ The inequality comes from the fact that $(a-b)^2\geq 0$ or can be thought as Cauchy-Schwarz applied to the vectors $\vec{x}=(1,1)$ and $\vec{y}=(a,b)$.