Question

If one of the lines given by 6x² - xy + 4cy² = 0 is 3x + 4y = 0, then c equals
(a) –3
(b) –1
(c) 3
(d) 1

Answer 1

3x+4y=0

3x=-4y

x=-4y/3. .............(1)

6x²+8xy-9xy+4cy²=0

2x(3x+4x)-y(9x-4cy)=0

2x x 0 -y(9x-4cy)=0

-y(9x-4cy)=0

9x-4cy=0

9x=4cy

From eq 1

9x(-4y)/3=4cy

3 x (-4y)=4cy

-12y=4cy

y is cancelled out then,

c =-12/4

i.e. c=-3

Anshwer is (-3)