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  • 一道積分問題:∫sinh((lnx))sin((lnx))dx

    一道積分問題:∫sinh((lnx))sin((lnx))dx
    如果不方便可以寫在紙上再上傳圖片,一樣可以接受,
    英語人氣:138 ℃時(shí)間:2020-04-16 09:55:32
    優(yōu)質(zhì)解答
    Q: ∫sinh((lnx))sin((lnx)) dx

    Let u = ln(x)
    ∴ du = (1/x) dx

    Note that e^(u)sinh((lnx))
    = e^(u) [ e^ln(x) - e^-ln(x) ] / 2
    = e^(u)(e^(u) - e^(-u)) /2
    = (1/2) (e^(2u) -1)

    ∫ sinh((lnx))sin((lnx)) dx
    = ∫ e^(u)sin(u)sinh(u)du (add in e^u, so you can utilize above formula)
    = ∫ (1/2)(e^(2u)-1)sin(u) du (substitute in above formula)
    = 1/2 ∫ (e^(2u)sin(u) - sin(u))du
    = 1/2 ∫ e^(2u)sin(u)du - 1/2 ∫ sin(u)du

    You have two choices here:

    1) Use integration by parts to solve the first integral, ∫ e^(2u)sin(u)du
    2) Use the formula, ∫ e^(αu)sin(βu)du = e^(αu)(-β cos(βu) + αsin(βu)) / (α²+ β²)、

    I chose the latter one.

    = 1/5 e^(2u)sin(u) - 1/10 e^(2u)cos(u) - 1/2 ∫ sin(u) u
    = 1/5 e^(2u)sin(u) - 1/10 e^(2u)cos(u) + cos(u)/2 + C

    Now substitude u = ln(x) back into the equation.

    = 1/5 x²sin(ln(x)) - 1/10 x²cos(ln(x)) + 1/2 cos(ln(x)) + C
    = (1/10)(2x²sin(ln(x)) - (x²-5)cos(ln(x)) + C

    ∴ (1/10)(2x²sin(ln(x)) - (x²-5)cos(ln(x)) + C
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