Example 4: Identify the given conic section as an ellipse, parabola, or hyperbola and find the equation of its directrix. \text { a) } r=\frac{7}{1+6 \sin \theta} \text { b) } r=\frac{3}{4-\cos \theta} \text { c) } r=\frac{1}{1+3 \cos \theta} \text { d) } r=\frac{5}{2+\cos \theta}

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