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$\widehat{AC}$ and $\widehat{BC}$ are arcs with centers $B$ and $A$ respectively. The circle in the figure passes through the midpoint of $AB$ and touches both the arcs.  If $AB=12$, find the radius of the circle.

 

 Feb 1, 2021
 #1
avatar+129840 
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Let AO = x + r

OC = r

AC = 6

 

By the Pythagorean Theorem

 

 

r^2 + 6^2  = (r + x) ^2

 

r^2 + 36 = r^2 + 2rx + x^2

 

36 = 2rx + x^2

 

x^2 + 2rx  - 36   = 0

 

x^2 + 2rx  =  36     complete the square on  x

 

x^2 + 2rx + r^2  = 36 +r^2

 

(x + r)^2  = 36 + r^2

 

x + r = sqrt [36 + r^2 ]

 

x = sqrt [36 + r^2 ] - r

 

x  + 2r  =  12

 

sqrt [ 36 +  r^2 ] - r + 2r  = 12

 

sqrt [ 36 +r^2] = 12 - r

 

36 + r^2 = r^2 -24r + 144

 

24r = 108

 

r =108 / 24 =   9/2

 

cool cool cool

 Apr 8, 2024

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