x^2 +(y-y2)^2 - R2^2 		= (x-x1)^2 + (y-y2)^2 -R1^2        		|T
x^2 + y^2 - 2yy2 + y2^2 - R2^2  = x^2 - 2xx1 + x1^2 + y^2 - 2yy1 + y1^2 + R1^2	| x^2,y^2 fallen weg
- 2yy2 + y2^2 - R2^2	 	= - 2xx1 + x1^2 - 2yy1 + y1^2 + R1^2		| +2yy1 -R2^2 -y2^2
+2yy1 - 2yy2			= - 2xx1 + x1^2 + y1^2 + R1^2 - R2^2 - y2^2	| T ( ausklammern )
-y(2y2 - 2y1)			= - 2xx1 + x1^2 + y1^2 + R1^2 - R2^2 - y2^2	| * (-1)
y(2y2 - 2y1)			=   2xx1 - x1^2 - y1^2 - R1^2 + R2^2 + y2^2     | /(2y2 - 2y1)


y				=   (2xx1 - x1^2 - y1^2 - R1^2 + R2^2 + y2^2) / (2y2 - 2y1)