求不定积分∫(2x∧2+x+1)/(x+3)(x-1)∧2dx

1个回答

  • 是不是想求:∫{(2x^2+x+1)/[(x+3)(x-1)^2]}dx  若是这样,则方法如下:

    原式=∫{[(2x^2-4x+2)+(5x-1)]/[(x+3)(x-1)^2]}dx

    =2∫[1/(x+3)]dx+∫{[(5x-5)+4]/[(x+3)(x-1)^2]}dx

    =2∫[1/(x+3)]d(x+3)+5∫[1/(x+3)(x-1)]dx

    +4∫[1/(x+3)(x-1)^2]dx

    2∫[1/(x+3)]d(x+3)+5∫[1/(x+3)(x-1)]dx

    +4∫[1/(x+3)(x-1)^2]d(x-1)

    =2ln|x+3|+(5/4)∫{[(x+3)-(x-1)]/(x+3)(x-1)]}dx

    -4∫[1/(x+3)]d[1/(x-1)]

    =2ln|x+3|+(5/4)∫[1/(x-1)]dx-(5/4)∫[1/(x+3)]dx

    -4∫[1/(x+3)]d[1/(x-1)]

    =2ln|x+3|+(5/4)∫[1/(x-1)]d(x-1)-(5/4)∫[1/(x+3)]d(x+3)

    -4∫[1/(x+3)]d[1/(x-1)]

    =2ln|x+3|+(5/4)ln|x-1|-(5/4)ln|x+3|-4∫[1/(x+3)]d[1/(x-1)]

    =(3/4)ln|x+3|+(5/4)ln|x-1|-4∫[1/(x+3)]d[1/(x-1)].

    下面求∫[1/(x+3)]d[1/(x-1)]的值

    令1/(x-1)=u,则:x-1=1/u,∴x=1+1/u=(u+1)/u,∴x+3=(4u+1)/u.

    ∴∫[1/(x+3)]d[1/(x-1)]

    =∫[u/(4u+1)]du

    =(1/4)∫[(4u+1-1)/(4u+1)]du

    =(1/4)∫du-(1/4)∫[1/(4u+1)]du

    =u/4-(1/16)∫[1/(4u+1)]d(4u+1)

    =1/[4(x-1)]-(1/16)ln|4u+1|+C

    =1/[4(x-1)]-(1/16)ln|4/(x-1)+1|+C

    =1/[4(x-1)]-(1/16)ln|(x+3)/(x-1)|+C

    =1/[4(x-1)]-(1/16)ln|x+3|+(1/16)ln|x-1|+C

    ∴原式=(3/4)ln|x+3|+(5/4)ln|x-1|

    -1/(x-1)+(1/4)ln|x+3|-(1/4)ln|x-1|+C

    =1/(1-x)+ln|x+3|+ln|x-1|+C

    注:若原题不是我所猜测的那样,则请你补充说明.