Geometrymedium · Past Paper
For the lines a1x + b1y + c1 = 0 and a2x + b2y + c2 = 0 to be parallel, the condition is:
Aa1/a2 = b1/b2
Ba1*a2 + b1*b2 = 0
Ca1*b2 = -a2*b1
Da1/b1 = a2/b2
✓ Correct Answer: A — a1/a2 = b1/b2
Parallel lines have the same slope: -a1/b1 = -a2/b2, which simplifies to a1/a2 = b1/b2.
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