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Exercise 10.4 · Q5

Q.Find the coordinates of the foci and the vertices, the eccentricity and the length of the latus rectum of the hyperbola 5y2−9x2=365y^2 - 9x^2 = 36.

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Rewriting 5y2−9x2=365y^2 - 9x^2 = 36 as y236/5−x24=1\dfrac{y^2}{36/5} - \dfrac{x^2}{4} = 1 (vertical transverse axis) gives a2=365, b2=4, c2=565a^2=\tfrac{36}{5},\ b^2=4,\ c^2=\tfrac{56}{5}: vertices (0,±65)\left(0,\pm\tfrac{6}{\sqrt5}\right), foci (0,±2145)\left(0,\pm\tfrac{2\sqrt{14}}{\sqrt5}\right), eccentricity 143\tfrac{\sqrt{14}}{3}, latus rectum 453\tfrac{4\sqrt5}{3}.

We are given the hyperbola 5y2−9x2=365y^2 - 9x^2 = 36 and must find its foci, vertices, eccentricity, and latus-rectum length. The plan is to bring the equation to standard form, read off a2a^2 and b2b^2, then use the standard hyperbola relations.

Step 1 — Convert to standard form

Divide both sides by 3636 so the right side becomes 11:

5y236−9x236=1  ⇒  y236/5−x24=1.\frac{5y^2}{36} - \frac{9x^2}{36} = 1 \;\Rightarrow\; \frac{y^2}{36/5} - \frac{x^2}{4} = 1.

The positive term is the y2y^2 term, so the transverse axis is vertical (along the y-axis). Comparing with y2a2−x2b2=1\dfrac{y^2}{a^2} - \dfrac{x^2}{b^2} = 1:

a2=365,b2=4,a^2 = \frac{36}{5}, \qquad b^2 = 4,

so a=65a = \dfrac{6}{\sqrt5} and b=2b = 2.

Note

For a vertical hyperbola the vertices and foci lie on the y-axis, at (0,±a)(0,\pm a) and (0,±c)(0,\pm c) — not on the x-axis.

Step 2 — Find cc

For a hyperbola, c2=a2+b2c^2 = a^2 + b^2:

c2=365+4=365+205=565.c^2 = \frac{36}{5} + 4 = \frac{36}{5} + \frac{20}{5} = \frac{56}{5}.

So

c=565=565=2145.c = \sqrt{\frac{56}{5}} = \frac{\sqrt{56}}{\sqrt5} = \frac{2\sqrt{14}}{\sqrt5}.

Step 3 — Vertices and foci

Vertices (0,±a)=(0,±65),\text{Vertices } (0, \pm a) = \left(0, \pm \frac{6}{\sqrt5}\right),

Foci (0,±c)=(0,±2145).\text{Foci } (0, \pm c) = \left(0, \pm \frac{2\sqrt{14}}{\sqrt5}\right).

Step 4 — Eccentricity

e=ca=214/56/5=2146=143.e = \frac{c}{a} = \frac{2\sqrt{14}/\sqrt5}{6/\sqrt5} = \frac{2\sqrt{14}}{6} = \frac{\sqrt{14}}{3}.

The 5\sqrt5 cancels, leaving e=143e = \dfrac{\sqrt{14}}{3} (which is >1>1, as it must be for a hyperbola). …

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