Difference between revisions of "1996 AHSME Problems/Problem 25"
Maxninga938 (talk | contribs) (→Solution 2B (Slightly less computation)) |
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<cmath>(x-7)^2 + (y-3)^2 = 64.</cmath> | <cmath>(x-7)^2 + (y-3)^2 = 64.</cmath> | ||
Applying Cauchy-Schwarz directly, | Applying Cauchy-Schwarz directly, | ||
− | <cmath>64 | + | <cmath>64\cdot25=(3^2+4^2)((x-7)^2 + (y-3)^2) \ge (3(x-7)+4(y-3))^2.</cmath> |
<cmath> 40 \ge 3x+4y-33 </cmath> | <cmath> 40 \ge 3x+4y-33 </cmath> | ||
<cmath>3x+4y \le 73.</cmath> | <cmath>3x+4y \le 73.</cmath> |
Revision as of 14:05, 8 July 2019
Contents
Problem
Given that , what is the largest possible value that
can have?
Solution 1
Complete the square to get
Applying Cauchy-Schwarz directly,
Thus our answer is
.
Solution 2 (Geometric)
The first equation is a circle, so we find its center and radius by completing the square:
, so
So we have a circle centered at with radius
, and we want to find the max of
.
The set of lines are all parallel, with slope
. Increasing
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 will be tangent to the circle.
Imagine that this line hits the circle at point . The slope of the radius connecting the center of the circle,
, to tangent point
will be
, since the radius is perpendicular to the tangent line.
So we have a point, , and a slope of
that represents the slope of the radius to the tangent point. Let's start at the point
. If we go
units up and
units right from
, we would arrive at a point that's
units away. But in reality we want
to reach the tangent point, since the radius of the circle is
.
Thus, , and we want to travel
up and
over from the point
to reach our maximum. This means the maximum value of
occurs at
, which is
Plug in those values for and
, and you get the maximum value of
, which is option
.
Solution 2B
Let the tangent point be , and the tangent line's x-intercept be
. Consider the horizontal line starting from center of circle (O) meeting the tangent line at K. Now triangle
is 3-4-5,
, so
. Note that the horizontal distance from
to the origin is
, and the horizontal distance from K to Q is 4, (
of its y coordinate), so the x-intercept is
. The value of
is 73 at point
. Note that this value is constant on the tangent line, so there is no need to calculate the coordinate of
.
.
Solution 3
Let . Solving for
, we get
. Substituting into the given equation, we get
which simplifies to
This quadratic equation has real roots in if and only if its discriminant is nonnegative, so
which simplifies to
which can be factored as
The largest value of
that satisfies this inequality is
, which is
.
Solution 4 (Using Answer Choice + Calculus)
Implicitly differentiating the given equation with respect to yields:
Now solve for to obtain:
Set the equation equal to zero to find the maximum occurs at
Plug this back into the equation that we are trying to maximize and see that we are left with:
.
The only answer choice that can be obtained from this equation is
See also
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 | ||
All AHSME Problems and Solutions |
The problems on this page are copyrighted by the Mathematical Association of America's American Mathematics Competitions.