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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
cos(40) = 4/x // - 4/x
cos(40)-(4/x) = 0
cos(40)-4*x^-1 = 0
-4*x^-1 = -cos(40) // : -4
x^-1 = cos(40)/4
-1 < 0
1/(x^1) = cos(40)/4 // * x^1
1 = (cos(40)/4)*x^1 // : cos(40)/4
4*(cos(40))^-1 = x^1
x = 4*(cos(40))^-1
x = 4*(cos(40))^-1
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