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D( a )
a = 0
a = 0
a = 0
a in (-oo:0) U (0:+oo)
20/a = -4/5 // + -4/5
20/a-(-4/5) = 0
20/a+4/5 = 0
20*a^-1 = -4/5 // : 20
a^-1 = -1/25
-1 < 0
1/(a^1) = -1/25 // * a^1
1 = -1/25*a^1 // : -1/25
-25 = a^1
a = -25
a = -25
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