Maths Olympiad Prep

Track / Stage 3 / 16 of 260 #16 of 1964

Problem 16

AMC 10/12, early questions
Geometry Difficulty 3.0 Multiple choice

Given the parabola x2=2yx^{2}=2y and its focus coincides with one of the foci of the ellipse y2m+x22=1\frac {y^{2}}{m}+ \frac {x^{2}}{2}=1, then m=m=

Pick one

Official solution

Solution: The focus of the parabola x2=2yx^{2}=2y is (0,12)(0, \frac {1}{2}), and one of the foci of the ellipse y2m+x22=1\frac {y^{2}}{m}+ \frac {x^{2}}{2}=1 is (0,m2)(0, \sqrt {m-2}). For them to coincide, we get m2=12\sqrt {m-2}= \frac {1}{2},
solving this gives m=94m= \frac {9}{4}.
Therefore, the correct choice is: D\boxed{D}.
To solve this problem, we find the coordinates of the focus of the parabola and ensure it coincides with the focus of the ellipse, then solve for mm.
This question tests the application of simple properties of ellipses and parabolas, as well as computational skills.

Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic, difficulty and ordering added by this site.