Tangent

how to find the eqn of common tangent btw 2 circles like in this question
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16 Replies
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@Apu
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Opt
Opt5mo ago
You can do this by the concept of centres of similitude can you not? C is just the external centre of similitude so it divides AB in ratio 2:1 externally
Gamertug
GamertugOP5mo ago
i have no idea what that word means similitude i tried assuming two point h k Alpha beta then the perp slope from center on tangets will be perp to line there were 4 eqn and 4 variable but damn it is near impossible to solve those equatins
Opt
Opt5mo ago
C divides AB externally in the ratio rA:rB is what you need to know I assume you got coords of B
Gamertug
GamertugOP5mo ago
ye i did
Opt
Opt5mo ago
Now it's just applying section formula
Gamertug
GamertugOP5mo ago
i mean we can do it without finding tangent and by method of similar triangle easily i just want to know how to find tangents if i ever have to in the future
Opt
Opt5mo ago
Take general tangent equation to one circle and impose condition that it is also true for the other, I guess. (y-yC) = m(x-xC)±r√(1+m²)
Gamertug
GamertugOP5mo ago
what is this? i only studied that T form for tangents xx1+yy1 + ....
Opt
Opt5mo ago
Conditions for tangency for conics? c =r√1+m² for circle c = a/m for parabola c = √a²m²+b² for ellipse c = √a²m²-b² for hyperbola ?
Gamertug
GamertugOP5mo ago
ooh that
SirLancelotDuLac
It would be better to approach this by geometry imo. Don't need the equation at all.
Gamertug
GamertugOP5mo ago
ye ik that , similar triangles , i just want to know if in future i want to find the eqn how to do it
iTeachChem
iTeachChem4mo ago
+solved @SirLancelotDuLac @Opt
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