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Find the combined equation of bisectors of angles between the lines represented by 5x2 + 6xy - y2 = 0. - Mathematics and Statistics

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Question

Find the combined equation of bisectors of angles between the lines represented by 5x2 + 6xy - y2 = 0.

Sum
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Solution

Comparing the equation 5x2 + 6xy - y2 = 0 with ax2 + 2hxy + by2 = 0, we get,

a = 5, 2h = 6, b = -1

Let m1 and m2 be the slopes of the lines represented by 5x2 + 6xy - y2 = 0.

∴ m1 + m2 = `(-2"h")/"b" = (-6)/-1 = 6`  and m1m2 = `"a"/"b" = 5/-1 = - 5`    ...(1)

The separate equations of the lines are

y = m1x and y = m2x, where m1 ≠ m2

i.e. m1x - y = 0 and m2x - y = 0.

Let P(x, y) be any point on one of the bisector of the angles between the lines.

∴ the distance of P from the line m1x - y = 0 is equal to the distance of P from the line m2x - y = 0.

∴ `|("m"_1"x" - "y")/(sqrt("m"_1^2 + 1))| = |("m"_2"x" - "y")/(sqrt("m"_2^2 + 1))|`

Squaring both sides, we get,

`("m"_1"x" - "y")^2/("m"_1^2 + 1) = ("m"_2"x" - "y")/("m"_2^2 + 1)`

∴ `("m"_2^2 + 1)("m"_1"x" - "y")^2 = ("m"_1^2 + 1)("m"_2"x" - "y")`

∴ `("m"_2^2 + 1)("m"_1^2"x"^2 - 2"m"_1"xy" + "y"^2) = ("m"_1^2 + 1)("m"_2^2"x"^2 - 2"m"_2"xy" + "y"^2)`


∴ `"m"_1^2"m"_2^2"x"^2 - 2"m"_1"m"_2^2"y"^2"xy" + "m"_2^2"y"^2 + "m"_1^2"x"^2 - 2"m"_1"xy" + "y"^2 = "m"_1^2"m"_2^2"x"^2 - 2"m"_1^2"m"_2"xy" + "m"_1^2"y"^2 + "m"_2^2"x"^2 - 2"m"_2"xy" + "y"^2`

∴ `("m"_1^2 - "m"_2^2)"x"^2 + 2"m"_1"m"_2("m"_1 -"m"_2)"xy" - 2("m"_1 - "m"_2)"xy" - ("m"_1^2 - "m"_2^2)"y"^2 = 0`

Dividing throughout by `"m"_1- "m"_2` (≠ 0), we get, 

`("m"_1 + "m"_2)"x"^2 + 2"m"_1"m"_2"xy" - 2"xy" - ("m"_1 + "m"_2)"y"^2 = 0`

∴ 6x2 - 10xy - 2xy - 6y2 = 0    ...[By (1)] 

∴ 6x2 - 12xy - 6y2 = 0 

∴ x2 - 2xy - y2 = 0

This is the joint equation of the bisectors of the angles between the lines represented by 5x2 + 6xy - y2 = 0.

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Combined Equation of a Pair Lines
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Chapter 4: Pair of Straight Lines - Miscellaneous Exercise 4 [Page 132]

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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 10 | Page 132

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