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If u and v ore differentiable functions of x. then prove that: ∫uv dx=u∫v dx-∫[dud∫v dx]dx Hence evaluate ∫logx dx - Mathematics and Statistics

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प्रश्न

If u and v ore differentiable functions of x. then prove that:

`int uv  dx = u intv  dx - int [(du)/(d) intv  dx]dx`

Hence evaluate `intlog x  dx`

योग

उत्तर

Let `intv  dx`= w   ......(i)

Then, `(dw)/(dx)` = v   ......(ii)

Now, `d/(dx)(u, w) = u. d/(dx)(w) + w d/(dx)(u)`

= `u.v + w  (du)/(dx)`  ......[From (ii)]

By definition of integration

u.w = `int[u.v + w . (du)/(dx)]dx`

∴ u.w = `intu.v  dx + intw.  (du)/(dx) dx`

∴ `intu.v  dx = u.w - intw.  (du)/(dx) dx`

= `uintv  dx - int[(du)/(dx) intv.  dx]dx`  ......[Using (i)]

Hence, `intlogx  dx = xlogx - int  1/x x xx dx`

= x log x – x + C

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