1996 AHSME Problems/Problem 25

Problem

Given that $x^2 + y^2 = 14x + 6y + 6$ , what is the largest possible value that $3x + 4y$ can have?

$\text{(A)}\ 72\qquad\text{(B)}\ 73\qquad\text{(C)}\ 74\qquad\text{(D)}\ 75\qquad\text{(E)}\ 76$

Solution 1

Complete the square to get $(x-7)^2 + (y-3)^2 = 64.$ Applying Cauchy-Schwarz directly, $64*25=(3^2+4^2)((x-7)^2 + (y-3)^2) \ge (3(x-7)+4(y-3))^2.$ $40 \ge 3x+4y-33$ $3x+4y \le 73.$ Thus our answer is $\boxed{(B)}$ .

Solution 2 (Geometric)

The first equation is a circle, so we find its center and radius by completing the square: $x^2 - 14x + y^2 - 6y = 6$ , so $(x-7)^2 + (y-3)^2 = (x- 14x + 49) + (y^2 - 6y + 9) = 6 + 49 + 9 = 64.$

So we have a circle centered at $(7,3)$ with radius $8$ , and we want to find the max of $3x + 4y$ .

The set of lines $3x + 4y = A$ are all parallel, with slope $-\frac{3}{4}$ . Increasing $A$ shifts the lines up and/or to the right.

We want to shift this line up high enough that it's tangent to the circle, but not so high that it misses the circle altogether. This means $3x + 4y = A$ will be tangent to the circle.

Imagine that this line hits the circle at point $(a,b)$ . The slope of the radius connecting the center of the circle, $(7,3)$ , to tangent point $(a,b)$ will be $\frac{4}{3}$ , since the radius is perpendicular to the tangent line.

So we have a point, $(7,3)$ , and a slope of $\frac{4}{3}$ that represents the slope of the radius to the tangent point. Let's start at the point $(7,3)$ . If we go $4k$ units up and $3k$ units right from $(7,3)$ , we would arrive at a point that's $5k$ units away. But in reality we want $5k = 8$ to reach the tangent point, since the radius of the circle is $8$ .

Thus, $k = \frac{8}{5}$ , and we want to travel $4\cdot \frac{8}{5}$ up and $3\cdot \frac{8}{5}$ over from the point $(7,3)$ to reach our maximum. This means the maximum value of $3x + 4y$ occurs at $\left(7 +3\cdot \frac{8}{5}, 3 + 4\cdot \frac{8}{5}\right)$ , which is $\left(\frac{59}{5}, \frac{47}{5}\right).$

Plug in those values for $x$ and $y$ , and you get the maximum value of $3x + 4y = 3\cdot\frac{59}{5} + 4\cdot\frac{47}{5} = \boxed{73}$ , which is option $\boxed{(B)}$ .

Solution 3

Let $z = 3x + 4y$ . Solving for $y$ , we get $y = (z - 3x)/4$ . Substituting into the given equation, we get $x^2 + \left( \frac{z - 3x}{4} \right)^2 = 14x + 6 \cdot \frac{z - 3x}{4} + 6,$ which simplifies to $25x^2 - (6z + 152)x + (z^2 - 24z - 96) = 0.$

This quadratic equation has real roots in $x$ if and only if its discriminant is nonnegative, so $(6z + 152)^2 - 4 \cdot 25 \cdot (z^2 - 24z - 96) \ge 0,$ which simplifies to $-64z^2 + 4224z + 32704 \ge 0,$ which can be factored as $-64(z + 7)(z - 73) \ge 0.$ The largest value of $z$ that satisfies this inequality is $\boxed{73}$ , which is $\boxed{(B)}$ .

Solution 4 (Using Answer Choice + Calculus)

Implicitly differentiating the given equation with respect to $x$ yields:

$2x + 2y\frac{dy}{dx} = 14 + 6\frac{dy}{dx}$

Now solve for $\frac{dy}{dx}$ to obtain:

$\frac{dy}{dx} = \frac{x - 7}{y - 3}$

Set the equation equal to zero to find the maximum occurs at $x = 0$

Plug this back into the equation that we are trying to maximize and see that we are left with: $21 + 4y$ .

The only answer choice that can be obtained from this equation is \fbox{ $73$ }

1996 AHSME (Problems • Answer Key • Resources)
Preceded by Problem 24	Followed by Problem 26
1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15 • 16 • 17 • 18 • 19 • 20 • 21 • 22 • 23 • 24 • 25 • 26 • 27 • 28 • 29 • 30
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