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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/(a-4) = a-a-1+5 // - a-a-1+5
a/(a-4)-a+a-5+1 = 0
a/(a-4)+(-a*(a-4))/(a-4)+(a*(a-4))/(a-4)+(-5*(a-4))/(a-4)+(1*(a-4))/(a-4) = 0
a*(a-4)-a*(a-4)-5*(a-4)+1*(a-4)+a = 0
a^2-a^2+5*a-4*a-5*a+a-4+20 = 0
a-5*a+a-4+20 = 0
a-4*a-4+20 = 0
16-3*a = 0
(16-3*a)/(a-4) = 0
(16-3*a)/(a-4) = 0 // * a-4
16-3*a = 0
16-3*a = 0 // - 16
-3*a = -16 // : -3
a = -16/(-3)
a = 16/3
a = 16/3
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