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D( x )
1-(4*x) = 0
1-(4*x) = 0
1-(4*x) = 0
1-4*x = 0
1-4*x = 0 // - 1
-4*x = -1 // : -4
x = -1/(-4)
x = 1/4
x in (-oo:1/4) U (1/4:+oo)
y = 2500/(1-(4*x)) // - 2500/(1-(4*x))
y-(2500/(1-(4*x))) = 0
y-2500*(1-4*x)^-1 = 0
y-2500/(1-4*x) = 0
(y*(1-4*x))/(1-4*x)-2500/(1-4*x) = 0
y*(1-4*x)-2500 = 0
y-4*x*y-2500 = 0
(y-4*x*y-2500)/(1-4*x) = 0
(y-4*x*y-2500)/(1-4*x) = 0 // * 1-4*x
y-4*x*y-2500 = 0
y-4*x*y-2500 = 0 // - y-2500
-4*x*y = -(y-2500) // : -4*y
x = (-(y-2500))/(-4*y)
x = (2500-y)/(-4*y)
x = (2500-y)/(-4*y)
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