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D( a )
2*a-4 = 0
2*a-4 = 0
2*a-4 = 0
2*a-4 = 0 // + 4
2*a = 4 // : 2
a = 4/2
a = 2
a in (-oo:2) U (2:+oo)
10/(2*a-4) = -1 // + 1
10/(2*a-4)+1 = 0
10/(2*a-4)+(1*(2*a-4))/(2*a-4) = 0
1*(2*a-4)+10 = 0
2*a+6 = 0
(2*a+6)/(2*a-4) = 0
(2*a+6)/(2*a-4) = 0 // * 2*a-4
2*a+6 = 0
2*a+6 = 0 // - 6
2*a = -6 // : 2
a = -6/2
a = -3
a = -3
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