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Question 54 of 65

Q.Evaluate ∫ sin(log x) dx. OR Evaluate ∫ (√(cot x) - √(tan x)) dx.

West Bengal WbchseWest Bengal HS (WBCHSE) Board 2024Subjective· 4mImportance★★★★★
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Substitute t=log⁡xt=\log x to turn this into the standard ∫etsin⁡t dt\int e^t\sin t\,dt form, which has a known formula.

Let t=log⁡xt = \log x, so x=etx = e^t and dx=et dtdx = e^t\,dt. The integral becomes:

I=∫sin⁡(log⁡x) dx=∫etsin⁡t dtI = \int \sin(\log x)\,dx = \int e^t\sin t\,dt

Using the standard formula ∫eatsin⁡(bt) dt=eat(asin⁡bt−bcos⁡bt)a2+b2+C\displaystyle\int e^{at}\sin(bt)\,dt = \frac{e^{at}(a\sin bt - b\cos bt)}{a^2+b^2}+C with a=1, b=1a=1,\,b=1:

∫etsin⁡t dt=et(sin⁡t−cos⁡t)2+C\int e^t\sin t\,dt = \frac{e^t(\sin t - \cos t)}{2}+C

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