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A parabola with vertex at the origin satisfies y squared equals 36x. What is the equation of its directrix?
Ax equals negative 36
By equals negative 9
Cx equals negative 9
Dx equals positive 9
Answer & Solution
Correct answer: C. x equals negative 9
1. The squared variable is y, so the form is y squared equals 4px and the axis is the x-axis.
2. Matching coefficients gives 4p equal to 36, so p is 9.
3. For this form the directrix is the vertical line x equals negative p.
4. So the directrix is x equals negative 9.
5. Answering x equals negative 36 uses the coefficient without dividing by 4.
_Source: OpenStax College Algebra (CC BY 4.0), Ch 8 "Analytic Geometry", section 8.3 The Parabola_
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