Sign in
Please select an account to continue using cracku.in
↓ →
Join Our JEE Preparation Group
Prep with like-minded aspirants; Get access to free daily tests and study material.
If $$e_1$$ and $$e_2$$ are the eccentricities of the ellipse $$\frac{x^2}{18} + \frac{y^2}{4} = 1$$ and the hyperbola $$\frac{x^2}{9} - \frac{y^2}{4} = 1$$ respectively and $$(e_1, e_2)$$ is a point on the ellipse $$15x^2 + 3y^2 = k$$, then the value of $$k$$ is equal to:
First, we note that for every conic of the form $$\frac{x^{2}}{a^{2}}+\frac{y^{2}}{b^{2}}=1$$ (ellipse with the major axis along the $$x$$-axis) the eccentricity formula is $$e=\sqrt{1-\frac{b^{2}}{a^{2}}}$$.
The given ellipse is $$\frac{x^{2}}{18}+\frac{y^{2}}{4}=1$$, so we identify $$a^{2}=18$$ and $$b^{2}=4$$ with $$a>b$$.
Applying the ellipse eccentricity formula, we write
$$e_{1}=\sqrt{1-\frac{b^{2}}{a^{2}}}=\sqrt{1-\frac{4}{18}}.$$
Simplifying the fraction inside the square root,
$$\frac{4}{18}=\frac{2}{9},$$ so
$$e_{1}=\sqrt{1-\frac{2}{9}}=\sqrt{\frac{9}{9}-\frac{2}{9}}=\sqrt{\frac{7}{9}}.$$
Taking the square root of a fraction, we get
$$e_{1}=\frac{\sqrt7}{3}.$$
Next we consider the hyperbola $$\frac{x^{2}}{9}-\frac{y^{2}}{4}=1$$. The standard form $$\frac{x^{2}}{a^{2}}-\frac{y^{2}}{b^{2}}=1$$ (transverse axis along $$x$$) has eccentricity formula $$e=\sqrt{1+\frac{b^{2}}{a^{2}}}$$. Here $$a^{2}=9$$ and $$b^{2}=4$$.
Thus
$$e_{2}=\sqrt{1+\frac{b^{2}}{a^{2}}}=\sqrt{1+\frac{4}{9}}.$$
We simplify inside the root:
$$1+\frac{4}{9}=\frac{9}{9}+\frac{4}{9}=\frac{13}{9},$$ so
$$e_{2}=\sqrt{\frac{13}{9}}=\frac{\sqrt{13}}{3}.$$
Because the ordered pair $$(e_{1},e_{2})$$ lies on the ellipse $$15x^{2}+3y^{2}=k$$, we substitute $$x=e_{1}$$ and $$y=e_{2}$$ into that equation.
Hence
$$15(e_{1})^{2}+3(e_{2})^{2}=k.$$
We already have
$$e_{1}^{2}=\left(\frac{\sqrt7}{3}\right)^{2}=\frac{7}{9},\qquad e_{2}^{2}=\left(\frac{\sqrt{13}}{3}\right)^{2}=\frac{13}{9}.$$
Substituting these squared values, we obtain
$$k=15\left(\frac{7}{9}\right)+3\left(\frac{13}{9}\right).$$
Multiplying numerators and denominators,
$$15\left(\frac{7}{9}\right)=\frac{105}{9},\qquad 3\left(\frac{13}{9}\right)=\frac{39}{9}.$$
Adding the two fractions,
$$k=\frac{105}{9}+\frac{39}{9}=\frac{144}{9}.$$
Finally, dividing numerator by denominator,
$$\frac{144}{9}=16.$$
Thus $$k=16$$.
Hence, the correct answer is Option A.
Create a FREE account and get:
Predict your JEE Main percentile, rank & performance in seconds
Educational materials for JEE preparation
Ask our AI anything
AI can make mistakes. Please verify important information.
AI can make mistakes. Please verify important information.