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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
800-((800/x-30)*(x-5)) = 60 // - 60
800-((800/x-30)*(x-5))-60 = 0
800-1*(800*x^-1-30)*(x-5)-60 = 0
800-1*(800*x^-1-30)*(x-5)-60 = 0
30*x+4000*x^-1-1010+800 = 0
30*x+4000*x^-1-210 = 0
30*x+4000*x^-1-210 = 0
10*(3*x+400*x^-1-21) = 0
3*x^1+400*x^-1-21*x^0 = 0
(3*x^2-21*x^1+400*x^0)/(x^1) = 0 // * x^2
x^1*(3*x^2-21*x^1+400*x^0) = 0
x^1
3*x^2-21*x+400 = 0
3*x^2-21*x+400 = 0
DELTA = (-21)^2-(3*4*400)
DELTA = -4359
DELTA < 0
x in { }
10 = 0
10 = 0
x belongs to the empty set
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