2021 AMC 12A Problems/Problem 7

Problem

What is the least possible value of $(xy-1)^2+(x+y)^2$ for real numbers $x$ and $y$ ?

$\textbf{(A)} ~0\qquad\textbf{(B)} ~\frac{1}{4}\qquad\textbf{(C)} ~\frac{1}{2} \qquad\textbf{(D)} ~1 \qquad\textbf{(E)} ~2$

Solution

Expanding, we get that the expression is $x^2+2xy+y^2+x^2y^2-2xy+1$ or $x^2+y^2+x^2y^2+1$ . By the trivial inequality(all squares are nonnegative) the minimum value for this is $\boxed{(D) 1}$ , which can be achieved at $x=y=0$ . ~aop2014

Solution 2 (Beyond Overkill)

Like solution 1, expand and simplify the original equation to $x^2+y^2+x^2y^2+1$ and let $f(x, y) = x^2+y^2+x^2y^2+1$ . To find local extrema, find where $\nabla f(x, y) = \boldsymbol{0}$ . First, find the first with respect to x and y and find where they are $0$ : $\frac{\partial f}{\partial x} = 2x + 2xy^{2} = 2x(1 + y^{2}) = 0 \implies x = 0$ $\frac{\partial f}{\partial y} = 2y + 2yx^{2} = 2y(1 + x^{2}) = 0 \implies y = 0$

Thus, there is a local extreme at $(0, 0)$ . Because this is the only extreme, we can assume that this is a minimum (though this can also be proven using the partial second derivative test) and the global minimum since it's the only minimum, meaning $f(0, 0)$ is the minimum of $f(x, y)$ . Plugging (0, 0) into f(x, y), we find 1 $\implies \boxed{\bold{(D)} \ 1}$

~ DBlack2021

Video Solution by Hawk Math

https://www.youtube.com/watch?v=P5al76DxyHY

2021 AMC 12A (Problems • Answer Key • Resources)
Preceded by Problem 6	Followed by Problem 8
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2021 AMC 12A Problems/Problem 7

Contents

Problem

Solution

Solution 2 (Beyond Overkill)

Video Solution by Hawk Math

See also