计算定积分∫[上限e^(π/2)下限1]sin(lnx)dx

1个回答

  • 设lnx=t,则当x=1时,t-0.当x=e^(π/2)时,t=π/2

    ∴原式=∫(0,π/2)e^tsintdt (∫(0,π/2)表示从0到π/2积分)

    为了求解方便,设I=∫(0,π/2)e^tsintdt

    ∵I=(e^tsintdt)|(0,π/2)-∫(0,π/2)e^tcostdt (应用分部积分)

    ==>I=e^(π/2)-(e^tcost)|(0,π/2)-∫(0,π/2)e^tcostdt (应用分部积分)

    ==>I=e^(π/2)+1-I (∵I=∫(0,π/2)e^tsintdt)

    ==>2I=e^(π/2)+1

    ==>I=[e^(π/2)+1]/2

    ∴原式=[e^(π/2)+1]/2.