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D( x )
21-x = 0
21-x = 0
21-x = 0
21-x = 0 // - 21
-x = -21 // * -1
x = 21
x in (-oo:21) U (21:+oo)
438.79 = 8337/(21-x) // - 8337/(21-x)
438.79-(8337/(21-x)) = 0
438.79-8337*(21-x)^-1 = 0
438.79-8337/(21-x) = 0
(438.79*(21-x))/(21-x)-8337/(21-x) = 0
438.79*(21-x)-8337 = 0
877.59-438.79*x = 0
(877.59-438.79*x)/(21-x) = 0
(877.59-438.79*x)/(21-x) = 0 // * 21-x
877.59-438.79*x = 0
877.59-438.79*x = 0 // - 877.59
-438.79*x = -877.59 // : -438.79
x = -877.59/(-438.79)
x = 2.00002279
x = 2.00002279
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