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D( a )
a-9 = 0
a-9 = 0
a-9 = 0
a-9 = 0 // + 9
a = 9
a in (-oo:9) U (9:+oo)
(a^2-(5*a)-42)/(a-9) = 0
(a^2-5*a-42)/(a-9) = 0
a^2-5*a-42 = 0
a^2-5*a-42 = 0
DELTA = (-5)^2-(-42*1*4)
DELTA = 193
DELTA > 0
a = (193^(1/2)+5)/(1*2) or a = (5-193^(1/2))/(1*2)
a = (193^(1/2)+5)/2 or a = (5-193^(1/2))/2
(a-((5-193^(1/2))/2))*(a-((193^(1/2)+5)/2)) = 0
((a-((5-193^(1/2))/2))*(a-((193^(1/2)+5)/2)))/(a-9) = 0
( a-((193^(1/2)+5)/2) )
a-((193^(1/2)+5)/2) = 0 // + (193^(1/2)+5)/2
a = (193^(1/2)+5)/2
( a-((5-193^(1/2))/2) )
a-((5-193^(1/2))/2) = 0 // + (5-193^(1/2))/2
a = (5-193^(1/2))/2
a in { (193^(1/2)+5)/2, (5-193^(1/2))/2 }
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