2014 AMC 12B Problems/Problem 17
Contents
Problem
Let be the parabola with equation and let . There are real numbers and such that the line through with slope does not intersect if and only if < < . What is ?
Solution 1 (Algebra Based)
Let . Equating them:
For there to be no solutions, the discriminant must be less than zero:
.
So for where and are the roots of and their sum by Vieta's formulas is .
Solution 2 (Calculus-based)
The line will begin to intercept the parabola when its slope equals that of the parabola at the point of tangency. Taking the derivative of the equation of the parabola, we get that the slope equals . Using the slope formula, we find that the slope of the tangent line to the parabola also equals . Setting these two equal to each other, we get Solving for , we get The sum of the two possible values for where the line is tangent to the parabola is , and the sum of the slopes of these two tangent lines is equal to , or .
Solution 3 (fake)
The smaller solution is basically negligible in comparison with the solution with the larger slope. Try some values like of where => , and slope ~80. Trying a few values leads us to conclude the least possible value is around , so the answer is .
~Arcticturn
See also
2014 AMC 12B (Problems • Answer Key • Resources) | |
Preceded by Problem 16 |
Followed by Problem 18 |
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 | |
All AMC 12 Problems and Solutions |
The problems on this page are copyrighted by the Mathematical Association of America's American Mathematics Competitions.