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D( a )
a = 0
a = 0
a = 0
a in (-oo:0) U (0:+oo)
2*a-(1/a) = 21/10 // - 21/10
2*a-(1/a)-(21/10) = 0
2*a-a^-1-21/10 = 0
2*a^1-1*a^-1-21/10*a^0 = 0
(2*a^2-21/10*a^1-1*a^0)/(a^1) = 0 // * a^2
a^1*(2*a^2-21/10*a^1-1*a^0) = 0
a^1
2*a^2+(-21/10)*a-1 = 0
2*a^2+(-21/10)*a-1 = 0
DELTA = (-21/10)^2-(-1*2*4)
DELTA = 1241/100
DELTA > 0
a = ((1241/100)^(1/2)-(-21/10))/(2*2) or a = (-(-21/10)-(1241/100)^(1/2))/(2*2)
a = ((1241/100)^(1/2)+21/10)/4 or a = (21/10-(1241/100)^(1/2))/4
a in { (21/10-(1241/100)^(1/2))/4, ((1241/100)^(1/2)+21/10)/4}
a in { (21/10-(1241/100)^(1/2))/4, ((1241/100)^(1/2)+21/10)/4 }
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