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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
4*y-10 = 900/x // - 900/x
4*y-(900/x)-10 = 0
4*y-900*x^-1-10 = 0
-900*x^-1 = -(4*y-10) // : -900
x^-1 = (10-(4*y))/(-900)
-1 < 0
1/(x^1) = (10-(4*y))/(-900) // * x^1
1 = x^1*((10-(4*y))/(-900)) // : (10-(4*y))/(-900)
-900*(10-4*y)^-1 = x^1
x = -900*(10-4*y)^-1
x = -900*(10-4*y)^-1
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