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geometry problem about circles

label Mathematics
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schedule 0 Hours
account_balance_wallet $5

how to find point of intersection of direct common tangents for two circles which touch each other?

Oct 19th, 2017

The Right Answer is:-

  • Start by drawing a diagram including the line joining the centres of the two circles and radii of both circles meeting the common tangent(s) at 90 degrees ( you only really need to draw one of the tangents ) 
  • It is handy to construct a line segment parallel to the line of centres joining the point of tangency in the smaller circle to the radius of the bigger circle forming a parallelogram and a right angled triangle, That triangle is similar to a triangle whose vertices include the point of intersection that you want to find ( clearly this point lies on the line of centres, )
  • This similarity allows you to calculate the distance from the point of intersection to the point of contact between the two circles.
  • ( Actually there are three points of intersection if you include the common tangent that passes through the point of contact - the count goes down to two if the circles have the same radius. )

Jan 18th, 2015

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