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Geometry Difficulty 3.6 AMC 10/12 Find the answer

The focus FF of the curve C_1C\_1: y2=2px(p>0)y^{2}=2px (p > 0) coincides with the right focus of the curve C_2C\_2: x2a2y2b2=1(a>0,b>0)\frac{x^{2}}{a^{2}}-\frac{y^{2}}{b^{2}}=1 (a > 0, b > 0), and the line connecting the intersection points of the curves C_1C\_1 and C_2C\_2 passes through point FF. What is the eccentricity of the curve C_2C\_2?

Pick one

Solution

The focus FF of the curve C_1C\_1: y2=2px(p>0)y^{2}=2px (p > 0) is F(p2,0)F(\frac{p}{2}, 0). Therefore, the value of cc for the hyperbola is p2\frac{p}{2}.

Since the line connecting the intersection points of the curves C_1C\_1 and C_2C\_2 passes through point FF, by symmetry, this line is perpendicular to the xx-axis. Let x=cx=c, and substitute it into the equation of the hyperbola:

y2=b2(c2a21)=b4a2 y^{2} = b^{2} \left( \frac{c^{2}}{a^{2}} - 1 \right) = \frac{b^{4}}{a^{2}}

Solving for yy, we get y=±b2ay = \pm \frac{b^{2}}{a}, and thus the length of the chord MN=2b2a|MN| = \frac{2b^{2}}{a}.

Now, let x=p2x = \frac{p}{2}, and substitute it into the equation of the parabola:

y2=p2 y^{2} = p^{2}

This gives us y=±py = \pm p, and thus the length of the chord MN=2p|MN| = 2p.

Equating the two expressions for MN|MN|, we get:

2p=2b2a 2p = \frac{2b^{2}}{a}

This simplifies to b2=2ac=c2a2b^{2} = 2ac = c^{2} - a^{2}. Solving for the eccentricity ee, we have:

e22e1=0 e^{2} - 2e - 1 = 0

Solving this quadratic equation for ee, we obtain e=1+2e = 1 + \sqrt{2}.

Answer: D:2+1\boxed{D: \sqrt{2} + 1}

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