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Q)

Find the joint equation of lines $x+y+1=0$ & $x+y-1=0$

$\begin{array}{1 1}(a)\;x^2+y^2+xy=1&(b)\;x^2+y^2+2xy=1\\(c)\;x^2+2y^2+xy=3&(d)\;x^2+y^2+3xy=1\end{array}$

$(x+y+1)(x+y-1)=0$
$(x+y)^2-1^2=0$
$x^2+y^2+2xy=1$