Explanation:
pythagoras theorem: c²=a²+b²
Where c= longest side of a right angle triangle
AB= 6+r = c
AD= 12 =a
CD= r =b
(6+r)² = 12²+ r²
36+12r+r² =144 +r²
12r+r²-r²=144-36
12r=108
r= 108/12
r=9
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