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D( a )
a+4 = 0
a+4 = 0
a+4 = 0
a+4 = 0 // - 4
a = -4
a in (-oo:-4) U (-4:+oo)
(((a+6)*(a+1))/(a+4))*(a-1) = 0
((a+6)*(a+1)*(a-1))/(a+4) = 0
( a+1 )
a+1 = 0 // - 1
a = -1
( a+6 )
a+6 = 0 // - 6
a = -6
( a-1 )
a-1 = 0 // + 1
a = 1
a in { -1, -6, 1 }
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