First slide

Pair of straight lines

Question

If the lines represented by $a x^{2} + 4 x y + y^{2} + 8 x + 2 f y + c = 0$ intersect on Y-axis, then (f, c) =

Moderate

A

(2,4)
B

(4,2)
C

(-2,-4)
D

(-4,-2)

Solution

$a = a, 2 h = 4, b = 1, 2 g = 8, 2 f = 2 f, c = c$
$h f - b g = 0 \Rightarrow f = 2$
$f^{2} = b \Rightarrow c = 4$

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