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D( x )
x = 0
x-12 = 0
x = 0
x = 0
x-12 = 0
x-12 = 0
x-12 = 0 // + 12
x = 12
x in (-oo:0) U (0:12) U (12:+oo)
12/x-(7/(x-12))+1 = 0
12/x-7*(x-12)^-1+1 = 0
12/x-7/(x-12)+1 = 0
(-7*x)/(x*(x-12))+(12*(x-12))/(x*(x-12))+(1*x*(x-12))/(x*(x-12)) = 0
12*(x-12)+1*x*(x-12)-7*x = 0
x^2+5*x-12*x-144 = 0
x^2-7*x-144 = 0
x^2-7*x-144 = 0
x^2-7*x-144 = 0
DELTA = (-7)^2-(-144*1*4)
DELTA = 625
DELTA > 0
x = (625^(1/2)+7)/(1*2) or x = (7-625^(1/2))/(1*2)
x = 16 or x = -9
(x+9)*(x-16) = 0
((x+9)*(x-16))/(x*(x-12)) = 0
((x+9)*(x-16))/(x*(x-12)) = 0 // * x*(x-12)
(x+9)*(x-16) = 0
( x+9 )
x+9 = 0 // - 9
x = -9
( x-16 )
x-16 = 0 // + 16
x = 16
x in { -9, 16 }
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