De Pythagore :
AE^2 + DE^2 = r^2
r^2 = (x+4)^2 ..............................(1)
r^2 = (12-x)^2+36
donc
(x+4)^2 = (12-x)^2+36
simplifier et calculer
x= 5
r = sqrt(9^2 + 4) (de (1))
= carré(85)
Solution sympa hein ?