# not so good at math, please help

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redbaseone    100
hello, im not so good at math, please help me, i need help calculating the 2-dimensional coordinate positions of the common tangent lines (l1 & l2 in image) of cirles C1 and C2, i.e the T1, T2 , T3 & T4 point coordinates like in this image: knowing following: the center of the circles C1 & C2 , also radii R & r and the distance "d" between centers of circles. thank you !

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Gage64    1235
This sounds like a homework question.

Are you allowed to use vector math? If you create certain vectors, then for some of them you'll have an equation for their length (which is given), and for others you can rely on the fact that the dot product of two perpendicular vectors is 0.

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http://www.geometrictools.com/LibFoundation/Miscellaneous/Miscellaneous.html

look at TangentsToCircles.

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redbaseone    100
i can use vector math.

thank you for replies,
i think i solved it.

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grhodes_at_work    1385
Quote:
 Original post by Gage64This sounds like a homework question.Are you allowed to use vector math? If you create certain vectors, then for some of them you'll have an equation for their length (which is given), and for others you can rely on the fact that the dot product of two perpendicular vectors is 0.

Gage64, based on redbaseone's prior, and recent, posts, I believe this is not homework. I believe he is working on a pathfinding/pathfollowing implementation for a game.

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