4. If z = ln (Y / x), then the value of the partial derivative of the function Z (x, y) at (1,2) is () A：0 B：1/2 C：1 D：-1/2

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• 17. Let f (x) = 2x + LNX & nbsp;, then & nbsp; & nbsp; & nbsp; & nbsp; & nbsp; () A. X = 12 is the maximum of F (x), B. x = 12 is the minimum of F (x), C. x = 2 is the maximum of & nbsp; f (x), D. x = 2 is the minimum of & nbsp; f (x)
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