Condition for a second-degree equation to represent a pair of lines: find the unknown

1. A.
   $fg=ch$
2. B.
   $gh=cf$
3. C.
   $fh=cg$
4. D.
   $hf=-cg$

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1. A.
   3
2. B.
   4
3. C.
   7
4. D.
   8

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1. A.
   $k = 5$ only
2. B.
   $k = 0$ only
3. C.
   $k = 0$ or $k = 5$
4. D.
   the value of k does not exist

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1. A.
   $1, 3$
2. B.
   $1, 2$
3. C.
   $1, -3$
4. D.
   $1, -2$

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