Question

In the $xy$ -plane, a circle of radius $4$ with center on the positive $x$ -axis is tangent to the $y$ -axis at the origin, and a circle with radius $10$ with center on the positive $y$ -axis is tangent to the $x$ -axis at the origin. What is the slope of the line passing through the two points at which these circles intersect?

$\dfrac{2}{7}$

$\dfrac{3}{7}$

$\dfrac{2}{\sqrt{29}}$

$\dfrac{1}{\sqrt{29}}$

$\dfrac{2}{5}$

Answer: E

Concepts:circle radical axis slope

Difficulty rating: 1440

Solution:

The circles are $(x-4)^2+y^2=16$ and $x^2+(y-10)^2=100,$ i.e. $x^2+y^2=8x$ and $x^2+y^2=20y.$ Subtracting gives $8x=20y,$ so the intersection points lie on $y=\tfrac{2}{5}x,$ which has slope $\tfrac{2}{5}.$

Thus, the correct answer is E.

2023 AMC 12B Problem 10

Answer: E

Difficulty rating: 1440

Solution:

Problem 10 in Other Years