∫ x * arctanx dx= ∫ arctanx d(x²/2)= (x²/2)arctanx - (1/2)∫ x² d(arctanx)= (x²/2)arctanx - (1/2)∫ x²/(x² + 1) dx= (x²/2)arctanx - (1/2)∫ (x² + 1 - 1)/(x
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