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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
cos(51) = 10/x // - 10/x
cos(51)-(10/x) = 0
cos(51)-10*x^-1 = 0
-10*x^-1 = -cos(51) // : -10
x^-1 = cos(51)/10
-1 < 0
1/(x^1) = cos(51)/10 // * x^1
1 = (cos(51)/10)*x^1 // : cos(51)/10
10*(cos(51))^-1 = x^1
x = 10*(cos(51))^-1
x = 10*(cos(51))^-1
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