The two lines $x=ay+b, z=cy+d;$ and $x=a'y+b' ,$ $z=c'y+d'$ are perpendicular to each other if :

Question

$aa'+cc'=-1$

$aa'+cc'=1$

${a \over {a'}} + {c \over {c'}} = - 1$

${a \over {a'}} + {c \over {c'}} = 1$

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