# Straight line touching two circles - fixed to another straight line

I want to find a series of steps or equations, that if possible can be put into a spreadsheet to solve the general case of the following problem. A straight line length $(a+b)$ touches two circles, radius $r_1$ centre $(x_1,y_1)$ and radius $r_2$ centre $(x_2,y_2)$. The point $O$ of this line is fixed to another straight line equation $y=mx+c$ such that length from this line to the first circle is always $a$ and from this line to the second circle is always $b$. I wish to get the coordinates $(x_a, y_a)$ and $(x_b, y_b)$ at where the line touches each circle and also $(x_O,y_O)$ at where it intersects $y=mx+c$. So far I have only managed to get answers by trial and error and not by drawing or mathematical calculation. The smaller circle is not always inside the larger circle.

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• Please attach a better image. The black background makes it almost impossible (for my old eyes) to decipher it. – zoli Mar 23 '17 at 22:23
• are the circle centers fixed? You have a four bar linkeage with a constraint that point O lies on a line. So the configuration is fully constrained with no degrees of freedom. Is this correct? – ja72 Mar 23 '17 at 22:31
• What is the interest of the line $y=mx+n$ and in particular the interest to have it described in this way (meaning of $m$ and $n$ ?). For me, it plays no role because it does not provide any new constraint on the problem. Am I right ? – Jean Marie Mar 23 '17 at 22:52
• Thankyou all. I have added a clearer image now. – Dez Mar 24 '17 at 14:02
• As drawn all parts are fixed. I need to do the calculation for the parts being in given fixed positions. (Known x1, y1, x2, y2, r1, r2, m, c, a and b). The parts drawn are moving parts and may be of different sizes hence the need for a general calculation. – Dez Mar 24 '17 at 14:13