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Let $$a > 0$$, $$b > 0$$. Let $$e$$ and $$l$$ respectively be the eccentricity and length of the latus rectum of the hyperbola $$\frac{x^2}{a^2} - \frac{y^2}{b^2} = 1$$. Let $$e'$$ and $$l'$$ respectively the eccentricity and length of the latus rectum of its conjugate hyperbola. If $$e^2 = \frac{11}{14}l$$ and $$(e')^2 = \frac{11}{8}l'$$, then the value of $$77a + 44b$$ is equal to
For the hyperbola $$\frac{x^2}{a^2} - \frac{y^2}{b^2} = 1$$:
Eccentricity: $$e^2 = 1 + \frac{b^2}{a^2}$$, Latus rectum: $$l = \frac{2b^2}{a}$$
For its conjugate hyperbola $$\frac{y^2}{b^2} - \frac{x^2}{a^2} = 1$$:
Eccentricity: $$(e')^2 = 1 + \frac{a^2}{b^2}$$, Latus rectum: $$l' = \frac{2a^2}{b}$$
Set up equations from the given conditions:
From $$e^2 = \frac{11}{14}l$$:
$$1 + \frac{b^2}{a^2} = \frac{11}{14} \cdot \frac{2b^2}{a} = \frac{11b^2}{7a}$$
$$\frac{a^2 + b^2}{a^2} = \frac{11b^2}{7a}$$
$$7a(a^2 + b^2) = 11a^2 b^2 \implies 7(a^2 + b^2) = 11ab^2 \quad \cdots (1)$$
From $$(e')^2 = \frac{11}{8}l'$$:
$$1 + \frac{a^2}{b^2} = \frac{11}{8} \cdot \frac{2a^2}{b} = \frac{11a^2}{4b}$$
$$\frac{a^2 + b^2}{b^2} = \frac{11a^2}{4b}$$
$$4b(a^2 + b^2) = 11a^2 b^2 \implies 4(a^2 + b^2) = 11a^2 b \quad \cdots (2)$$
Find the ratio $$b/a$$:
Dividing equation (1) by equation (2):
$$\frac{7}{4} = \frac{11ab^2}{11a^2 b} = \frac{b}{a}$$
Therefore $$b = \frac{7a}{4}$$.
Solve for $$a$$:
Substituting $$b = \frac{7a}{4}$$ into equation (2):
$$4\left(a^2 + \frac{49a^2}{16}\right) = 11a^2 \cdot \frac{7a}{4}$$
$$4 \cdot \frac{16a^2 + 49a^2}{16} = \frac{77a^3}{4}$$
$$\frac{65a^2}{4} = \frac{77a^3}{4}$$
$$65 = 77a \implies a = \frac{65}{77} = \frac{5}{7} \cdot \frac{13}{11}$$
Find $$b$$ and compute $$77a + 44b$$:
$$b = \frac{7}{4} \cdot \frac{65}{77} = \frac{7 \times 65}{4 \times 77} = \frac{65}{44}$$
$$77a + 44b = 77 \times \frac{65}{77} + 44 \times \frac{65}{44} = 65 + 65 = 130$$
The correct answer is Option D: $$130$$.
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