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Chapter 11 Class 11 Conic Sections
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Misc 1 - If parabolic reflector is 20 cm in diameter, 5 cm deep - Miscellaneous

Misc 1 - Chapter 11 Class 11 Conic Sections - Part 2
Misc 1 - Chapter 11 Class 11 Conic Sections - Part 3

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Misc 1 If a parabolic reflector is 20 cm in diameter and 5 cm deep, find the focus. Let equation of parabola be y2 = 4ax Here we need to find a (focus) Hence, Taking Point A and finding its coordinates Parabola is 5 cm deep Hence OC = 5 cm So, x-coordinate of point A = 5 It can be seen from figure AB = 20 cm Since parabola is symmetric along x-axis AC = 1 2 AB = 10 cm Hence coordinates of point A is (5 , 10) Since point A(5 , 10) lies on the parabola, it will satisfy the equation of the parabola Putting x = 5 and y = 10 in (1) y2 = 4ax 102 = 4 5 100 = 20a 100 20 = a 5 = a a = 5 Thus, Focus is (a , 0) = (5 , 0)

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Davneet Singh has done his B.Tech from Indian Institute of Technology, Kanpur. He has been teaching from the past 12 years. He provides courses for Maths, Science, Social Science, Physics, Chemistry, Computer Science at Teachoo.