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D( r )
r = 0
r = 0
r = 0
r in (-oo:0) U (0:+oo)
4*r-(4/r) = 8/7 // - 8/7
4*r-(4/r)-(8/7) = 0
4*r-4*r^-1-8/7 = 0
4*r^1-4*r^-1-8/7*r^0 = 0
(4*r^2-8/7*r^1-4*r^0)/(r^1) = 0 // * r^2
r^1*(4*r^2-8/7*r^1-4*r^0) = 0
r^1
4*r^2+(-8/7)*r-4 = 0
4*r^2+(-8/7)*r-4 = 0
DELTA = (-8/7)^2-(-4*4*4)
DELTA = 3200/49
DELTA > 0
r = ((3200/49)^(1/2)-(-8/7))/(2*4) or r = (-(-8/7)-(3200/49)^(1/2))/(2*4)
r = ((3200/49)^(1/2)+8/7)/8 or r = (8/7-(3200/49)^(1/2))/8
r in { (8/7-(3200/49)^(1/2))/8, ((3200/49)^(1/2)+8/7)/8}
r in { (8/7-(3200/49)^(1/2))/8, ((3200/49)^(1/2)+8/7)/8 }
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