求帮做几道高数题

1.设t=arcsinx，则：x=sint.
∫ x arcsinx dx = ∫ sint t d(sint) =∫ t sint cost dt = 1/2 ∫ t sin2t dt
=-1/4 ∫ t d(cos2t) = -1/4(t cos2t - ∫ cos2t dt)
=-1/4 t cos2t + 1/4 ∫ cos2t dt=-1/4 t cos2t + 1/8 sin2t
=-1/4 arcsinx cos(2 arcsinx) + 1/8 sin(2 arcsinx)

2.=x*ln(x+(x^2+1)^(1/2))-↑xd(ln(x+(x^2+1)^(1/2))
=xln(x+(x^2+1)^(1/2))-↑x*1/(x+(x^2+1)^(1/2))*(1+1/2*1/(x^2+1)^(1/2)*2x)dx
=xln(x+(x^2+1)^(1/2))-↑x/(x^2+1)^(1/2)dx
=xln(x+(x^2+1)^(1/2))-1/2↑1/(x^2+1)^(1/2)d(x^2+1)
=xln(x+(x^2+1)^(1/2))-(x^2+1)^(1/2)+C

第3题题目是什么意思?