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D( x )
x = 0
x = 0
x = 0
x in (-oo:0) U (0:+oo)
21/15 = 7/x // - 7/x
21/15-(7/x) = 0
21/15-7*x^-1 = 0
-7*x^-1 = -7/5 // : -7
x^-1 = 1/5
-1 < 0
1/(x^1) = 1/5 // * x^1
1 = 1/5*x^1 // : 1/5
5 = x^1
x = 5
x = 5
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