(4) (int )_(0)^+infty dfrac (dx)((1+x)(1+{x)^2)}

(int )_(0)^+infty dfrac (x{e)^-x}({(1+{e)^-x)}^2}dx= ()-|||-__

4.下列结论正确的是 ()-|||-(A) (int )_(-infty )^+infty dfrac (x)(1+{x)^2}dx=0 (B) (int )_

[例3] (int )_(0)^+infty dfrac (x{e)^-x}({(1+{e)^-x)}^2}dx

()-|||-(2) (int )_(-infty )^+infty dfrac (2x)(1+{x)^2}dx=lim _(aarrow +infty )(i

int dfrac ({x)^3}({(1+{x)^2)}^2}dx=A.int dfrac ({x)^3}({(1+{x)^2)}^2}dx=B.int df

(B)| (x/(1+2)^2 (1+x^2)^2 dx=0.-|||-(C) (int )_(-1)^1dfrac (1)(sin x)dx=0. (D) (

(int )_(-1)^1(dfrac (sin x+1)(1+{x)^2}+(x)^4)dxA (int )_(-1)^1(dfrac (sin x+1)(

10.若 (int )_(-infty )^+infty dfrac (A)(1+{x)^2}dx=1 则常数A等于:-|||-

( 单选题 ) int dfrac ({e)^x}(1+{e)^2x}dx=_________int dfrac ({e)^x}(1+{e)^2x}dx=int

已知(x)=dfrac (xcos x)(1+{x)^4}+(x)^2-(int )_(-1)^1f(x)dx,则(x)=dfrac (xcos x)(1+{x