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D( x )
18-x = 0
18-x = 0
18-x = 0
18-x = 0 // - 18
-x = -18 // * -1
x = 18
x in (-oo:18) U (18:+oo)
0.65 = 11/(18-x) // - 11/(18-x)
0.65-(11/(18-x)) = 0
0.65-11*(18-x)^-1 = 0
0.65-11/(18-x) = 0
(0.65*(18-x))/(18-x)-11/(18-x) = 0
0.65*(18-x)-11 = 0
0.7-0.65*x = 0
(0.7-0.65*x)/(18-x) = 0
(0.7-0.65*x)/(18-x) = 0 // * 18-x
0.7-0.65*x = 0
0.7-0.65*x = 0 // - 0.7
-0.65*x = -0.7 // : -0.65
x = -0.7/(-0.65)
x = 1.07692308
x = 1.07692308
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