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D( x )
5*x-20 = 0
4-x = 0
5*x-20 = 0
5*x-20 = 0
5*x-20 = 0 // + 20
5*x = 20 // : 5
x = 20/5
x = 4
4-x = 0
4-x = 0
4-x = 0 // - 4
-x = -4 // * -1
x = 4
x in (-oo:4) U (4:+oo)
8/(4-x)+x/(5*x-20) = 0
(8*(5*x-20))/((4-x)*(5*x-20))+(x*(4-x))/((4-x)*(5*x-20)) = 0
8*(5*x-20)+x*(4-x) = 0
44*x-x^2-160 = 0
44*x-x^2-160 = 0
44*x-x^2-160 = 0
DELTA = 44^2-(-160*(-1)*4)
DELTA = 1296
DELTA > 0
x = (1296^(1/2)-44)/(-1*2) or x = (-1296^(1/2)-44)/(-1*2)
x = 4 or x = 40
(x-4)*(x-40) = 0
((x-4)*(x-40))/((4-x)*(5*x-20)) = 0
((x-4)*(x-40))/((4-x)*(5*x-20)) = 0 // * (4-x)*(5*x-20)
(x-4)*(x-40) = 0
( x-40 )
x-40 = 0 // + 40
x = 40
( x-4 )
x-4 = 0 // + 4
x = 4
x in { 4}
x = 40
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