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D( r )
r = 0
r^2 = 0
r = 0
r = 0
r^2 = 0
r^2 = 0
1*r^2 = 0 // : 1
r^2 = 0
r = 0
r in (-oo:0) U (0:+oo)
4-(7/r)-(2/(r^2)) = 0
4-7*r^-1-2*r^-2 = 0
t_1 = r^-1
4-2*t_1^2-7*t_1^1 = 0
4-2*t_1^2-7*t_1 = 0
DELTA = (-7)^2-(-2*4*4)
DELTA = 81
DELTA > 0
t_1 = (81^(1/2)+7)/(-2*2) or t_1 = (7-81^(1/2))/(-2*2)
t_1 = -4 or t_1 = 1/2
t_1 = -4
r^-1+4 = 0
1*r^-1 = -4 // : 1
r^-1 = -4
-1 < 0
1/(r^1) = -4 // * r^1
1 = -4*r^1 // : -4
-1/4 = r^1
r = -1/4
t_1 = 1/2
r^-1-1/2 = 0
1*r^-1 = 1/2 // : 1
r^-1 = 1/2
-1 < 0
1/(r^1) = 1/2 // * r^1
1 = 1/2*r^1 // : 1/2
2 = r^1
r = 2
r in { -1/4, 2 }
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