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D( a )
a+15 = 0
a-9 = 0
a+15 = 0
a+15 = 0
a+15 = 0 // - 15
a = -15
a-9 = 0
a-9 = 0
a-9 = 0 // + 9
a = 9
a in (-oo:-15) U (-15:9) U (9:+oo)
(4*a)/(a+15) = 4/(a-9) // - 4/(a-9)
(4*a)/(a+15)-(4/(a-9)) = 0
(4*a)/(a+15)-4*(a-9)^-1 = 0
(4*a)/(a+15)-4/(a-9) = 0
(4*a*(a-9))/((a+15)*(a-9))+(-4*(a+15))/((a+15)*(a-9)) = 0
4*a*(a-9)-4*(a+15) = 0
4*a^2-40*a-60 = 0
4*a^2-40*a-60 = 0
4*(a^2-10*a-15) = 0
a^2-10*a-15 = 0
DELTA = (-10)^2-(-15*1*4)
DELTA = 160
DELTA > 0
a = (160^(1/2)+10)/(1*2) or a = (10-160^(1/2))/(1*2)
a = (4*10^(1/2)+10)/2 or a = (10-4*10^(1/2))/2
4*(a-((10-4*10^(1/2))/2))*(a-((4*10^(1/2)+10)/2)) = 0
(4*(a-((10-4*10^(1/2))/2))*(a-((4*10^(1/2)+10)/2)))/((a+15)*(a-9)) = 0
(4*(a-((10-4*10^(1/2))/2))*(a-((4*10^(1/2)+10)/2)))/((a+15)*(a-9)) = 0 // * (a+15)*(a-9)
4*(a-((10-4*10^(1/2))/2))*(a-((4*10^(1/2)+10)/2)) = 0
( a-((10-4*10^(1/2))/2) )
a-((10-4*10^(1/2))/2) = 0 // + (10-4*10^(1/2))/2
a = (10-4*10^(1/2))/2
( a-((4*10^(1/2)+10)/2) )
a-((4*10^(1/2)+10)/2) = 0 // + (4*10^(1/2)+10)/2
a = (4*10^(1/2)+10)/2
a in { (10-4*10^(1/2))/2, (4*10^(1/2)+10)/2 }
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