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Find the joint equation of the pair of a line through the origin and perpendicular to the lines given by 2x2 - 3xy - 9y2 = 0

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Question

Find the joint equation of the pair of a line through the origin and perpendicular to the lines given by

2x2 - 3xy - 9y2 = 0

Sum
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Solution

Comparing the equation 2x2 - 3xy - 9y2 = 0 with ax2 + 2hxy + by2 = 0, we get,

a = 2, 2h = - 3, b = - 9

Let m1 and m2 be the slopes of the lines represented by 2x2 - 3xy - 9y2 = 0 

∴ m1 + m2 = `(-"2h")/"b" = -3/9`  and  m1m2 = `"a"/"b" = -2/9`    ...(1)

Now, required lines are perpendicular to these lines

∴ their slopes are `(-1)/"m"_1` and `- 1/"m"_2`

Since these lines are passing through the origin, their separate equations are

y = `(-1)/"m"_1 "x"` and y = `(-1)/"m"_2 "x"`

i.e. m1y = - x and m2y = - x

i.e. x + m1y  = 0 and x + m2y = 0

∴ their combined equation is

(x + m1y)(x + m2y) = 0

∴ x2 + (m1 + m2)xy + m1m2y2 = 0

∴ `"x"^2 + (-3/9) "xy" + (-2/9)"y"^2 = 0`   ....[By(1)]

∴ `9"x"^2 - 3"xy" - 2"y"^2 = 0`

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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 4.2 | Page 131

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