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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
cos(20) = 7/x // - 7/x
cos(20)-(7/x) = 0
cos(20)-7*x^-1 = 0
-7*x^-1 = -cos(20) // : -7
x^-1 = cos(20)/7
-1 < 0
1/(x^1) = cos(20)/7 // * x^1
1 = (cos(20)/7)*x^1 // : cos(20)/7
7*(cos(20))^-1 = x^1
x = 7*(cos(20))^-1
x = 7*(cos(20))^-1
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