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D( a )
a-16 = 0
a+4 = 0
a-16 = 0
a-16 = 0
a-16 = 0 // + 16
a = 16
a+4 = 0
a+4 = 0
a+4 = 0 // - 4
a = -4
a in (-oo:-4) U (-4:16) U (16:+oo)
-((4*a)/(a+4)) = 12/(a-16) // - 12/(a-16)
-((4*a)/(a+4))-(12/(a-16)) = 0
-4*a*(a+4)^-1-12*(a-16)^-1 = 0
(-4*a)/(a+4)-12/(a-16) = 0
(-4*a*(a-16))/((a+4)*(a-16))+(-12*(a+4))/((a+4)*(a-16)) = 0
-4*a*(a-16)-12*(a+4) = 0
52*a-4*a^2-48 = 0
52*a-4*a^2-48 = 0
4*(13*a-a^2-12) = 0
13*a-a^2-12 = 0
DELTA = 13^2-(-12*(-1)*4)
DELTA = 121
DELTA > 0
a = (121^(1/2)-13)/(-1*2) or a = (-121^(1/2)-13)/(-1*2)
a = 1 or a = 12
4*(a-1)*(a-12) = 0
(4*(a-1)*(a-12))/((a+4)*(a-16)) = 0
(4*(a-1)*(a-12))/((a+4)*(a-16)) = 0 // * (a+4)*(a-16)
4*(a-1)*(a-12) = 0
( a-1 )
a-1 = 0 // + 1
a = 1
( a-12 )
a-12 = 0 // + 12
a = 12
a in { 1, 12 }
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