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D( a )
a+12 = 0
a-18 = 0
a+12 = 0
a+12 = 0
a+12 = 0 // - 12
a = -12
a-18 = 0
a-18 = 0
a-18 = 0 // + 18
a = 18
a in (-oo:-12) U (-12:18) U (18:+oo)
-((3*a)/(a+12)) = 9/(a-18) // - 9/(a-18)
-((3*a)/(a+12))-(9/(a-18)) = 0
-3*a*(a+12)^-1-9*(a-18)^-1 = 0
(-3*a)/(a+12)-9/(a-18) = 0
(-3*a*(a-18))/((a+12)*(a-18))+(-9*(a+12))/((a+12)*(a-18)) = 0
-3*a*(a-18)-9*(a+12) = 0
45*a-3*a^2-108 = 0
45*a-3*a^2-108 = 0
3*(15*a-a^2-36) = 0
15*a-a^2-36 = 0
DELTA = 15^2-(-36*(-1)*4)
DELTA = 81
DELTA > 0
a = (81^(1/2)-15)/(-1*2) or a = (-81^(1/2)-15)/(-1*2)
a = 3 or a = 12
3*(a-3)*(a-12) = 0
(3*(a-3)*(a-12))/((a+12)*(a-18)) = 0
(3*(a-3)*(a-12))/((a+12)*(a-18)) = 0 // * (a+12)*(a-18)
3*(a-3)*(a-12) = 0
( a-12 )
a-12 = 0 // + 12
a = 12
( a-3 )
a-3 = 0 // + 3
a = 3
a in { 12, 3 }
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