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The lines (y - b) = m1(x + a) and (y - b) = m2(x + a) are both tangents to the parabola y² = 4ax. What is the relation between m1 and m2?
- m1 + m2 = 0
- m1*m2 = 1
- m1*m2 = -1
- m1 + m2 = 1
Correct answer: m1*m2 = -1
Solution
The point (-a, b) lies on the directrix x = -a; the pair of tangents from a directrix point are perpendicular, so m1*m2 = -1.
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