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D( x )
x = 0
x-3 = 0
x = 0
x = 0
x-3 = 0
x-3 = 0
x-3 = 0 // + 3
x = 3
x in (-oo:0) U (0:3) U (3:+oo)
40/x-(8/7) = 20/(x-3) // - 20/(x-3)
40/x-(20/(x-3))-(8/7) = 0
40/x-20*(x-3)^-1-8/7 = 0
40/x-20/(x-3)-8/7 = 0
(-20*7*x)/(7*x*(x-3))+(7*40*(x-3))/(7*x*(x-3))+(-8*x*(x-3))/(7*x*(x-3)) = 0
7*40*(x-3)-8*x*(x-3)-20*7*x = 0
140*x-8*x^2+24*x-840 = 0
164*x-8*x^2-840 = 0
164*x-8*x^2-840 = 0
4*(41*x-2*x^2-210) = 0
41*x-2*x^2-210 = 0
DELTA = 41^2-(-210*(-2)*4)
DELTA = 1
DELTA > 0
x = (1^(1/2)-41)/(-2*2) or x = (-1^(1/2)-41)/(-2*2)
x = 10 or x = 21/2
4*(x-10)*(x-21/2) = 0
(4*(x-10)*(x-21/2))/(7*x*(x-3)) = 0
(4*(x-10)*(x-21/2))/(7*x*(x-3)) = 0 // * 7*x*(x-3)
4*(x-10)*(x-21/2) = 0
( 4 )
4 = 0
x belongs to the empty set
( x-10 )
x-10 = 0 // + 10
x = 10
( x-21/2 )
x-21/2 = 0 // + 21/2
x = 21/2
x in { 10, 21/2 }
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