2014 AMC 12B Problems/Problem 17

Problem 17

Let $P$ be the parabola with equation $y=x^2$ and let $Q = (20, 14)$ . There are real numbers $r$ and $s$ such that the line through $Q$ with slope $m$ does not intersect $P$ if and only if $r$ < $m$ < $s$ . What is $r + s$ ?

$\textbf{(A)}\ 1\qquad\textbf{(B)}\ 26\qquad\textbf{(C)}\ 40\qquad\textbf{(D)}}\ 52\qquad\textbf{(E)}\ 80$ (Error compiling LaTeX. Unknown error_msg)

Solution (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 $2x$ . Using the slope formula, we find that the slope of the tangent line to the parabola also equals $\frac{14-x^2}{20-x}$ . Setting these two equal to each other, we get $2x = \frac{14-x^2}{20-x} \implies x^2-40x+14 = 0$ Solving for $x$ , we get $x= 20\pm \sqrt{386}$ The sum of the two possible values for $x$ where the line is tangent to the parabola is $40$ , and the sum of the slopes of these two tangent lines is equal to $2x$ , or $\boxed{\textbf{(E)}\ 80}$ .

(Solution by kevin38017)

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2014 AMC 12B Problems/Problem 17

Problem 17

Solution (Calculus-based)