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The question is why does the line with equation ax+by+c=0 has a negative slop is wrong? I get a and b<0 by -b/2a , is it correct to proof the values in this way? However, when I try to calculate its slope by -a/b, it is a negative slope. I have no idea why the above statement is wrong. Can anyone please teach me? Thank you![enter image description here]1

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  • $\begingroup$ I get a and b<0 by -b/2a Could you explain that step by step. Think you've got a sign wrong. $\endgroup$ – dxiv Dec 4 '16 at 4:12
  • $\begingroup$ If you have a and b smaller than 0, when calculating slope by -a/b the slope will definitely be negative. This is my idea $\endgroup$ – Estudiantekriss Dec 4 '16 at 4:30
  • $\begingroup$ Question was how did you get a and b smaller than 0 in the first place. $\endgroup$ – dxiv Dec 4 '16 at 4:32
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$a<0$ because of downward parabola.
$\frac{-2b}{2a}>0 \implies b>0$ by the $X$ co-ordinate of vertex. Now figure out the sign of slope of the straight line.
P.S. : You don't actually need sign of $a$ to figure out the sign of slope.

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