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Question 23 of 39

Q.Evaluate the following : ∫x3⋅log⁡x dx\int x^3 \cdot \log x \, dx

Maharashtra MsbshseMaharashtra HSC (MSBSHSE) Board 2023Subjective· 3mImportance★★★★★
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Use integration by parts ∫u dv=uv−∫v du\int u\,dv = uv - \int v\,du with u=log⁡xu = \log x and dv=x3 dxdv = x^3\,dx (so v=x44v = \tfrac{x^4}{4}). The remaining integral is elementary.

Choose (by the LIATE rule, the logarithm is the first function):

u=log⁡x,dv=x3 dx ⇒ du=1x dx,v=x44.u = \log x,\quad dv = x^3\, dx \ \Rightarrow\ du = \frac{1}{x}\, dx,\quad v = \frac{x^4}{4}.

Apply ∫u dv=uv−∫v du\int u\, dv = uv - \int v\, du:

∫x3log⁡x dx=log⁡x⋅x44−∫x44⋅1x dx=x44log⁡x−14∫x3 dx.\int x^3 \log x\, dx = \log x \cdot \frac{x^4}{4} - \int \frac{x^4}{4}\cdot\frac{1}{x}\, dx = \frac{x^4}{4}\log x - \frac{1}{4}\int x^3\, dx.

Evaluate the last integral: …

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