Misc 2 - If l1, m1, n1 and l2, m2, n2 are direction cosines of two

Misc 2 - Chapter 11 Class 12 Three Dimensional Geometry - Part 2

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Question 2 If 𝑙_1 , π‘š_1, 𝑛_1 and 𝑙_2 , π‘š_2, 𝑛_2 are the direction cosines of two mutually perpendicular lines, show that direction cosines of line perpendicular to both of these are π‘š_1 𝑛_2 – π‘š_2 𝑛_1 , 𝑛_1 𝑙_2 – 𝑛_2 𝑙_1 , 𝑙_1 π‘š_2 – 𝑙_2 π‘š_1.We know that π‘Ž βƒ— Γ— 𝑏 βƒ— is perpendicular to both π‘Ž βƒ— & 𝑏 βƒ— So, required line is cross product of lines having direction cosines 𝑙_1 , π‘š_1, 𝑛_1 and 𝑙_2 , π‘š_2, 𝑛_2 Required line = |β– 8(𝑖 Μ‚&𝑗 Μ‚&π‘˜ Μ‚@𝑙_1&π‘š_1&𝑛_1@𝑙_2&π‘š_2&𝑛_2 )| = 𝑖 Μ‚ (π‘š_1 𝑛_2 – π‘š_2 𝑛_1) – 𝑗 Μ‚ (𝑙_1 𝑛_2 – 𝑙_2 𝑛_1) + π‘˜ Μ‚(𝑙_1 π‘š_2 – 𝑙_2 π‘š_1) = (π‘š_1 𝑛_2 – π‘š_2 𝑛_1) 𝑖 Μ‚ + (𝑙_2 𝑛_1βˆ’π‘™_1 𝑛_2) 𝑗 Μ‚ + (𝑙_1 π‘š_2 – 𝑙_2 π‘š_1) π‘˜ Μ‚ Hence, direction cosines = π‘š_1 𝑛_2 – π‘š_2 𝑛_1 , 𝑛_1 𝑙_2 – 𝑛_2 𝑙_1 , 𝑙_1 π‘š_2 – 𝑙_2 π‘š_1 ∴ Direction cosines of the line perpendicular to both of these are π‘š_1 𝑛_2 – π‘š_2 𝑛_1 , 𝑛_1 𝑙_2 – 𝑛_2 𝑙_1 , 𝑙_1 π‘š_2 – 𝑙_2 π‘š_1. Hence proved

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Davneet Singh has done his B.Tech from Indian Institute of Technology, Kanpur. He has been teaching from the past 13 years. He provides courses for Maths, Science, Social Science, Physics, Chemistry, Computer Science at Teachoo.