Definite integral

2011-12-20 2:16 am
Evaluate d/dα ∫(α->α²) sin(αx) / x dx,where α>0

I have two Methods

Method1:
dL/dα ∫(α->α²) sin(αx) / x dx
= sin(α³) / α² * (2α) - sin(α²) / α * 1
= 2sin(α³) / α - sin(α²) / α
= (1/α)[2sin(α³) - sin(α²)]

Method2:
L(α) = ∫(α,α²) sin(αx) / x dx
Let t = αx,dt = αdx => dx = (1/α)dt
When x = α,t = α²;whenx = α²,t =α³
L(α) = ∫(α²,α³) sin(t) / (t/α) * (1/α)dt
= ∫(α²,α³) sin(t) / t dt
= dL/dα = sin(α³)/α³ * (3α²) - sin(α²)/α² * (2α)
= 3sin(α³) / α - 2sin(α²) / α
= (1/α)[3sin(α³) - 2sin(α²)]

The answers obtained from two ways are different,
I want to ask which Method is correct?

回答 (1)



收錄日期: 2021-04-16 13:48:04
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