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两圆公切线长的计算，无论是外公切线长的计算还是内公切线长的计算，都归结为直角三角形的计算．计算外公切线长时，直角三角形由圆心距和两圆半径的差确定；计算内公切线长时，直角三角形由圆心距和两圆半径的和确定．辅助线的作法是：连结两圆过切点的半径，再过其中一圆的?
The calculation of the tangent length of the two circles, whether it is the calculation of the length of the tangent of the grandfather or the calculation of the tangent length of the male, all comes down to the calculation of a right triangle. When calculating the tangent length of the grandfather, the right-angle triangle is determined by the difference between the circle center distance and the radius of the two circles; when calculating the length of the tangent, the right-angle triangle is determined by the sum of the circle center distance and the radius of the two circles. The practice of auxiliary lines is: to connect the two circles through the radius of the cut point, and then pass one of the circles?