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Exercise 10.4 · Q4

Q.Find the coordinates of the foci and the vertices, the eccentricity and the length of the latus rectum of the hyperbola 16x2−9y2=57616x^2 - 9y^2 = 576.

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This hyperbola is in standard form x2a2−y2b2=1\frac{x^2}{a^2} - \frac{y^2}{b^2} = 1 after dividing by 576. We find a=6a = 6, b=8b = 8, then c=a2+b2=10c = \sqrt{a^2 + b^2} = 10. The foci are (±10,0)(\pm 10, 0), vertices (±6,0)(\pm 6, 0), eccentricity e=53e = \frac{5}{3}, and latus rectum length 2b2a=643\frac{2b^2}{a} = \frac{64}{3}.

The given equation is 16x2−9y2=57616x^2 - 9y^2 = 576. This is a hyperbola, and the first step is always to rewrite it in the standard form so we can read off the key parameters.

Why standard form? The standard forms for a hyperbola centered at the origin are:

  • Horizontal transverse axis: x2a2−y2b2=1\frac{x^2}{a^2} - \frac{y^2}{b^2} = 1
  • Vertical transverse axis: y2a2−x2b2=1\frac{y^2}{a^2} - \frac{x^2}{b^2} = 1

Here, the x2x^2 term is positive and the y2y^2 term is negative, so it's the horizontal case. The numbers aa and bb give us the vertices, and c=a2+b2c = \sqrt{a^2 + b^2} gives the foci. Eccentricity e=c/ae = c/a, and latus rectum length is 2b2/a2b^2/a for a horizontal hyperbola.

Let's work through it.

  1. Divide through by 576 to get the right-hand side equal to 1.

16x2576−9y2576=1\frac{16x^2}{576} - \frac{9y^2}{576} = 1

Simplify each fraction:

x236−y264=1\frac{x^2}{36} - \frac{y^2}{64} = 1

  1. Identify a2a^2 and b2b^2. From x2a2−y2b2=1\frac{x^2}{a^2} - \frac{y^2}{b^2} = 1, we have a2=36a^2 = 36 and b2=64b^2 = 64. So:

a=6,b=8a = 6, \quad b = 8

  1. Find cc. For a hyperbola, c2=a2+b2c^2 = a^2 + b^2 (not a2−b2a^2 - b^2 — that's for ellipses). So:

c2=36+64=100⇒c=10c^2 = 36 + 64 = 100 \quad \Rightarrow \quad c = 10

Watch out

A common mistake is to use c2=a2−b2c^2 = a^2 - b^2 (the ellipse formula). For hyperbolas, it's always c2=a2+b2c^2 = a^2 + b^2 because c>ac > a.

  1. Vertices. For a horizontal hyperbola, vertices are at (±a,0)=(±6,0)(\pm a, 0) = (\pm 6, 0).

  2. Foci. Foci are at (±c,0)=(±10,0)(\pm c, 0) = (\pm 10, 0). …

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