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what is reduction formula in integration?explain it

Krish Mishra , 14 Years ago
Grade Upto college level
anser 1 Answers
Ashwin Sinha

Last Activity: 14 Years ago

Dear Krish Mishra,

reduction formula is one that enables us to solve an integral problem by reducing it to a problem of solving an easier integral problem, and then reducing that to the problem of solving an easier problem, and so on.

For example, if we let

I_n = \int x^n e^x\,dx

Integration by parts allows us to simplify this to

I_n = x^ne^x - n\int x^{n-1}e^x\,dx=
I_n = x^ne^x - nI_{n-1} \,\!

which is our desired reduction formula. Note that we stop at

I_0 = e^x \,\!.

Similarly, if we let

I_n = \int \sec^n \theta \, d\theta

then integration by parts lets us simplify this to

I_n = \sec^{n-2}\theta \tan \theta -  (n-2)\int \sec^{n-2} \theta \tan^2 \theta \, d\theta

Using the trigonometric identity, tan2θ = sec2θ − 1, we can now write

\begin{matrix} I_n & = & \sec^{n-2}\theta \tan \theta &  + (n-2) \left( \int \sec^{n-2} \theta \, d\theta - \int \sec^n \theta \, d\theta \right) \\ & = & \sec^{n-2}\theta \tan \theta & + (n-2) \left( I_{n-2}  - I_n \right) \\ \end{matrix}

Rearranging, we get

I_n=\frac{1}{n-1}\sec^{n-2}\theta \tan \theta + \frac{n-2}{n-1} I_{n-2}

Note that we stop at n = 1 or 2 if n is odd or even respectively.

As in these two examples, integrating by parts when the integrand contains a power often results in a reduction formula.

 

                                    Best Of Luck............

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