∫xln(x+1)dx＝∫ln(x+1)d(1/2*x^2)＝1/2×x^2×ln(x+1)－1/2×∫x^2dln(x+1)＝1/2×x^2×ln(x+1)－1/2×∫x^2/(x+1)dx＝1/2×x^2×ln(x+1)－1/2×∫[x-1+1/(x+1)]dx＝1/2×x^2×ln(x+1)－1/2×[1/2×x^2－x＋ln(x+1...