Pair of Straight Lines

Q.

If the equation
$a{x}^2+2hxy+b{y}^2+2gx+2fy+c=0$
represents a pair of straight lines, then
$abc+2fgh-a{f}^2-b{g}^2+c{h}^2=0$

1. True

2. False

Q.

Which among the following equations represents a pair of straight lines?

1. $x^2-y^2+x-y+1=0$

2. $x^2-2x-y+3=0$

3. $x^2+3xy+2y^2-x-4y-6=0$

4. $3x^2-4xy-7y^2=0$