Question

(a) Show that the asymptotes of the hyperbola

x2y2=5x ^ { 2 } - y ^ { 2 } = 5

are perpendicular to each other. (b) Find an equation for the hyperbola with foci

(±c,0)( \pm c ,0 )

and with asymptotes perpendicular to each other.

Solution

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Answered 2 years ago
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(a) x2y2=5    x25y25=1x^2-y^2=5\;\to\;\dfrac{x^2}{5}-\dfrac{y^2}{5}=1 is hyperbola of the form x2a2y2b2=1\dfrac{x^2}{a^2}-\dfrac{y^2}{b^2}=1 where a2=b2=5    a=b=5a^2=b^2=5\;\to\;a=b=\sqrt5.

Its asymptotes are then of the form y=±bax    y=±55x    y=±xy=\pm\dfrac{b}{a}x\;\to\;y=\pm\dfrac{\sqrt5}{\sqrt5}x\;\to\;y=\pm x.

Recall that perpendicular lines have slopes with a product of 1-1. Since y=xy=x has a slope of 1, y=xy=-x has a slope of 1-1, and the product of the slopes is 1(1)=11(-1)=-1, then x2y2=5x^2-y^2=5 has perpendicular asymptotes.

(b) If the foci are (±c,0)(\pm c,0), then the equation must be of the form x2a2y2b2=1\dfrac{x^2}{a^2}-\dfrac{y^2}{b^2}=1 where c2=a2+b2c^2=a^2+b^2 and the asymptotes are y=±baxy=\pm\dfrac{b}{a}x.

The slopes of the asymptotes are ab\dfrac{a}{b} and ab-\dfrac{a}{b} so if they are perpendicular, then ab(ab)=1    a2b2=1    a2=b2-\dfrac{a}{b}\left(\dfrac{a}{b}\right)=-1\;\to\;\dfrac{a^2}{b^2}=1\;\to\;a^2=b^2.

We then get:

c2=a2+b2    c2=a2+a2    c2=2a2    a2=c22c^2=a^2+b^2\;\to\;c^2=a^2+a^2\;\to\;c^2=2a^2\;\to\;a^2=\dfrac{c^2}{2}

Therefore, b2=a2=c22b^2=a^2=\dfrac{c^2}{2} so the equation is:

x2a2y2b2=1    x2c2/2y2c2/2=1    x2y2=c22\dfrac{x^2}{a^2}-\dfrac{y^2}{b^2}=1\;\to\;\dfrac{x^2}{c^2/2}-\dfrac{y^2}{c^2/2}=1\;\to\;\boxed{x^2-y^2=\dfrac{c^2}{2}}

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