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D( a )
a = 0
a = 0
a = 0
a in (-oo:0) U (0:+oo)
1/5-((4/5)*a) = 4/a // - 4/a
1/5-((4/5)*a)-(4/a) = 0
(-4/5)*a-4*a^-1+1/5 = 0
1/5*a^0-4/5*a^1-4*a^-1 = 0
(1/5*a^1-4/5*a^2-4*a^0)/(a^1) = 0 // * a^2
a^1*(1/5*a^1-4/5*a^2-4*a^0) = 0
a^1
(-4/5)*a^2+(1/5)*a-4 = 0
(-4/5)*a^2+(1/5)*a-4 = 0
DELTA = (1/5)^2-(-4*4*(-4/5))
DELTA = -319/25
DELTA < 0
a in { }
a belongs to the empty set
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