Proposed Problem
See also:Complete Problem 28, Collection of Geometry Problems
Level: High School, SAT Prep, College geometry
Saturday, April 11, 2009
Problem 28. Right Triangle, altitude, incircles, inradius
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Let incircle of Tr. ABC meet AB in H. Thus, AE = AH = AB - BH = c - r = c - (c+a-b)/2 = (c-a+b)/2.
ReplyDeleteFrom Tr ABD, AD = c^2/b. Hence, DE = AE - AD = (c-a+b)/2 -c^2/b = (bc -ab+b^2-2c^2)/2 ------(1)
Triangles ABD & CBD give 2r2=CD+BD -a and 2r1= AD+BD -c whence 2r2 -2r1 = CD - AD + c - a = a^2/b - c^2/b + c -a or r2-r1=(bc-ab+a^2-c^2)/2b = (bc-ab+b^2-2c^2)/2---(2) since b^2=a^2+c^2. Equations (1) & (2) give us r2 -r1 = DE whereas it is appaent from the diagram that FG =r2 - r1. Hence, DE = FG = r2 - r1
Vihaan: vihaanup@gmail.com
AB=a , BC =b , AD= n , DC= m and BD = h
ReplyDeleteFor the radius of the circles we have the following equations :
2r=a+b-m-n
2r1 = n+h-a
2r2 =m+h-b
DC= m and EC= b-r so we get :
DE =DC-EC=m-(b-r)
using the formula for 2r :
2DE=2m-2b+2r
2DE=2m-2b+a+b-n-m
2DE=m-b+a-n
2DE=m+h-b+a-h-n
2DE=m+h-b-(n+h-a)
m+h-b=2r2 and n+h-a =2r1 therefore we get :
2DE=2r2-2r1
DE=r2-r1
How to find the inradius of tr. ABC?
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