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D( a )
2*a = 0
2*a = 0
2*a = 0
2*a = 0 // : 2
a = 0
a in (-oo:0) U (0:+oo)
1 = 15/(2*a) // - 15/(2*a)
1-(15/(2*a)) = 0
(-15/2)*a^-1+1 = 0
-15/2*a^-1 = -1 // : -15/2
a^-1 = 2/15
-1 < 0
1/(a^1) = 2/15 // * a^1
1 = 2/15*a^1 // : 2/15
15/2 = a^1
a = 15/2
a = 15/2
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