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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
40/100 = 168/x // - 168/x
40/100-(168/x) = 0
40/100-168*x^-1 = 0
-168*x^-1 = -2/5 // : -168
x^-1 = 1/420
-1 < 0
1/(x^1) = 1/420 // * x^1
1 = 1/420*x^1 // : 1/420
420 = x^1
x = 420
x = 420
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