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**C.C** It's a equation for finding all tangent lines to a circle when the point isn't on the circle itself. I think I remember that it was

d=m(x)+b-(y)/√1+m^2

But I can't remember if the x and y are in the correct place.

Edit- Let me show an example. Find the equation of all tangent lines that can be drawn from the point (5,0) to the circle (x-0)^2+(y-0)^2=9.

First I find the line equation.

y-0=m(x-5)

y=m(x-5)+0

y=mx-5m+0

Then I plug this into the equation, note I can only do this one because the center is at the origin because I still can't remember if the x or y are in the correct place but since they are both 0, it doesn't matter for me.

3= m(0)-5m+0-(0)/√1+m^2

9= 25m^2/1+m^2

9+9m^2=25m^2

0=14m^2-9

Uhhh then I blank out here... So I guess I need help solving it and remembering the equation correctly...