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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
1/5 = (x-12)/x // - (x-12)/x
1/5-((x-12)/x) = 0
(-1*(x-12))/x+1/5 = 0
(-1*5*(x-12))/(5*x)+(1*x)/(5*x) = 0
1*x-1*5*(x-12) = 0
60-4*x = 0
(60-4*x)/(5*x) = 0
(60-4*x)/(5*x) = 0 // * 5*x
60-4*x = 0
60-4*x = 0 // - 60
-4*x = -60 // : -4
x = -60/(-4)
x = 15
x = 15
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