The derivative of arctan(x) = 1 / (1 + x^2). Using the chain rule, this needs to be multiplied by the derivative of (x ln(x)) as follows:
(1 / (1 + x^2)) [x(1/x) + ln(x))] = (1 + ln(x)) / (1 + x^2) <== answer
(1 / (1 + x^2)) [x(1/x) + ln(x))] = (1 + ln(x)) / (1 + x^2) <== answer
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diff(arctan(x*ln(x)), x) = (ln(x) + 1)/(1 + x^2*ln(x)^2)