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Find k, if one of the lines given by 3x2 - kxy + 5y2 = 0 is perpendicular to the line 5x + 3y = 0.

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Question

Find k, if one of the lines given by 3x2 - kxy + 5y2 = 0 is perpendicular to the line 5x + 3y = 0.

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Solution

The auxiliary equation of the lines given by 3x2 - kxy + 5y2 = 0 is  5m2 - km + 3 = 0

Now, one line is perpendicular to the line 5x + 3y = 0, whose slope is `- 5/3`

∴ slope of that line = m = `3/5`

∴ m = `3/5` is the root of the auxiliary equation 5m2 - km + 3 = 0

∴ `5(3/5)^2 - "k"(3/5) + 3 = 0`

∴ `9/5 - "3k"/5 + 3 = 0`

∴ 9 - 3k + 15 = 0

∴ 3k = 24

∴ k = 8

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Chapter 4: Pair of Straight Lines - Miscellaneous Exercise 4 [Page 131]

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Balbharati Mathematics and Statistics 1 (Arts and Science) [English] Standard 12 Maharashtra State Board
Chapter 4 Pair of Straight Lines
Miscellaneous Exercise 4 | Q 5.4 | Page 131

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