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In calculus, integration by parametric derivatives, also called parametric integration,[1] is a method of Using known Integrals to integrate derived functions. It is often used in Physics, and is similar to integration by substitution.
By using The Leibniz integral rule with the upper and lower bounds fixed we get that
It is also true for non-finite bounds.
Examples[]
Example One: Exponential Integral[]
For example, suppose we want to find the integral
Since this is a product of two functions that are simple to integrate separately, repeated integration by parts is certainly one way to evaluate it. However, we may also evaluate this by starting with a simpler integral and an added parameter, which in this case is t = 3:
This converges only for t > 0, which is true of the desired integral. Now that we know
we can differentiate both sides twice with respect to t (not x) in order to add the factor of x2 in the original integral.
This is the same form as the desired integral, where t = 3. Substituting that into the above equation gives the value:
Example Two: Gaussian Integral[]
Starting with the integral
And Taking the derivative with respect to t of both sides
In general, Taking the n-th derivative with respect to t gives us
Example Three: A Polynomial[]
Using the classical and taking the derivative with respect to t We get
Example Four: Sums[]
The method can also b used on sums.
Using the Weierstrass factorization of the sinh function
Taking a logarithm
Taking The derivative with respect to z Letting