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Geometry Difficulty 3.8 AMC 10/12 Find the answer

Given the hyperbola x29y216=1\frac {x^{2}}{9} - \frac {y^{2}}{16} = 1 with its left and right foci marked as F1 and F2 respectively. If there is a point P on the hyperbola such that the angle ∠F1PF2 is 90°, find the area of triangle ΔF1PF2, denoted as SF1PF2S_{△F_{1}PF_{2}}.

A number or a short expression. Spacing and $ signs are ignored.

Solution

Since the equation of the hyperbola is given by x29y216=1\frac {x^{2}}{9} - \frac {y^{2}}{16} = 1, we have semi-major axis a=3a = 3 and semi-minor axis b=4b = 4. The distance from the center to a focus can be calculated using c=a2+b2=9+16=5c = \sqrt{a^2+b^2} = \sqrt{9+16} = 5.

According to the definition of a hyperbola, the absolute differences in distances from any point on a hyperbola to the foci is constant and equal to 2a2a. Therefore, PF1PF2=±2a=±6|PF_{1}| - |PF_{2}| = \pm2a = \pm6. Squaring both sides,
PF122PF1PF2+PF22=36|PF_{1}|^2 - 2|PF_{1}|\cdot|PF_{2}| + |PF_{2}|^2 = 36.

This equation can be rearranged to give:
PF12+PF22=36+2PF1PF2|PF_{1}|^2 + |PF_{2}|^2 = 36 + 2|PF_{1}|\cdot|PF_{2}|.

Since we are given that the angle ∠F1PF2 is 90°, it follows from the Pythagorean theorem that:
PF12+PF22=F1F22=c2=25|PF_{1}|^2 + |PF_{2}|^2 = |F_{1}F_{2}|^2 = c^2 = 25.

Combining these two equations, we have:
100=36+2PF1PF2100 = 36 + 2|PF_{1}|\cdot|PF_{2}|,
which simplifies to:
PF1PF2=32|PF_{1}|\cdot|PF_{2}| = 32.

The area of triangle ΔF1PF2 is given by one-half of the product of the lengths of the two segments forming the right angle, so:
SF1PF2=12(PF1PF2)=12(32)=16S_{△F_{1}PF_{2}} = \frac{1}{2}(|PF_{1}|\cdot|PF_{2}|) = \frac{1}{2}(32) = 16.

Therefore, the area of the triangle is 16\boxed{16}.

The problem is essentially asking us to find the product of the distances from the point P to each focus and then calculate the area of the triangle using the fact that there's a right angle formed at P. The key in solving this problem is to use the definition of a hyperbola and apply the Pythagorean theorem effectively.

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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.