The value of k for which the lines 5x + 7y =3 and 15x + 21y = k coincide is

(a) 9   (b) 5   (c) 7   (d) 18

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Question 2 The value of k for which the lines 5x + 7y =3 and 15x + 21y = k coincide is (a) 9 (b) 5 (c) 7 (d) 18 Given equations 5x + 7y = 3 15x + 21y = k 5x + 7y – 3 = 0 Comparing with a1x + b1y + c1 = 0 ∴ a1 = 5 , b1 = 7 , c1 = –3 15x + 21y − k = 0 Comparing with a2x + b2y + c2 = 0 ∴ a2 = 15 , b2 = 21 , c2 = –k Now, 𝑎1/𝑎2 = 5/15 𝑎1/𝑎2 = 1/3 𝑏1/𝑏2 = 7/21 𝑏1/𝑏2 = 1/3 𝑐1/𝑐2 = (−3)/(−𝑘) 𝑐1/𝑐2 = 3/𝑘 Since lines are coincident 𝑎1/𝑎2 = 𝑏1/𝑏2 = 𝑐1/𝑐2 𝟏/𝟑 = 𝟏/𝟑 = 𝟑/𝒌 Thus, 1/3 = 3/𝑘 k = 3 × 3 k = 9 So, the correct answer is (a)

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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.