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## 'Three Ways' printed from http://nrich.maths.org/

Given that $x + y = -1$ find the largest value of $xy$

(a) by co-ordinate geometry

(b) by calculus

(c) by algebra.

Here are some solutions from Koopa,Boston College, USA. Vassil,
Lawnswood Sixth Form, Leeds sent in similar methods. Can you find a
co-ordinate geometry (i.e. graphical) method or yet another
different method?

Method 1

We have $x + y = -1$. So, to maximise $xy$, I need to maximize

\[-x(x + 1) = -[(x + 1/2)^2 - 1/4] = -(x + 1/2)^2 + 1/4,\]

so, $xy$ is maximized at $x = -1/2$ and the maximum value is $1/4$.

Method 2

Let $f(x) = -x(x + 1)$, then by differentiation $f'(x) = -2x - 1$
and to find a maximum or minimum $f'(x)= 0$ gives $x = -1/2$. The
second derivative test easily verifies that this indeed gives a
maximum so the maximum value is $1/4$.

Method 3

By the AM-GM inequality, we have

\[(xy)^{(1/2)} \leq (x + y)/2\]

so \[xy \leq (-1/2)^2 = 1/4.\]