7
QuizMultiple Choice

Equations of Hyperbolas (continued) — Quiz

Question 7 • Precalculus

A hyperbola has its foci at open paren 7 comma 5 close paren and open paren 7 comma negative 5 close paren A directrix of the hyperbola is y is equal to 16 fifths What is the equation of the hyperbola?

Answer
A
the fraction with numerator y squared and denominator 16 minus the fraction with numerator open paren x minus 7 close paren squared and denominator 9 is equal to 1
B
the fraction with numerator x squared and denominator 16 minus the fraction with numerator open paren y minus 7 close paren squared and denominator 9 is equal to 1
C
the fraction with numerator x squared and denominator 9 minus the fraction with numerator open paren y minus 7 close paren squared and denominator 16 is equal to 1
D
the fraction with numerator y squared and denominator 9 minus the fraction with numerator open paren x minus 7 close paren squared and denominator 16 is equal to 1

Explanation

The foci have midpoint (7, 0), so the center is (7, 0), and the hyperbola opens vertically because the foci differ in their y-coordinates. For a vertical hyperbola, the directrix is y = k + a²/c; here 16/5 = a²/5, so a² = 16. Since c² = a² + b² and c = 5, b² = 25 − 16 = 9. Thus the equation is y²/16 − (x − 7)²/9 = 1, which is choice A.

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