Answers>Maths>IB>Article

What is integration by parts, and how is it useful?

"Integration by parts" is one of several methods in our arsenal that we can use to integrate a function f(x). It involves expressing f(x) as the product of two other functions, which I will call u(x) and v'(x) (where v' = dv/dx is the first derivative of v with respect to x):
 
∫ f(x) dx = ∫ u(x) · v'(x) dx =  u(x) · v(x) - ∫ u'(x) · v(x) dx
 
This formula comes from the product rule for differentiation. If we differentiate the product u(x) · v(x) with respect to x using the product rule, we get the following:
 
d/dx(u(x) · v(x)) = u(x) · v'(x) + u'(x) · v(x)
 
Integrating both sides of this expression with respect to x removes the d/dx on the left hand side, and creates two integrals on the right hand side:
 
u(x) · v(x) = ∫ u(x) · v'(x) dx + ∫ u'(x) · v(x) dx
 
If we subtract the final term from both sides, we arrive at our original formula again:
 
∫ f(x) dx = ∫ u(x) · v'(x) dx =  u(x) · v(x) - ∫ u'(x) · v(x) dx
 
But why would we want to use this method? Well, integration by parts is generally useful in cases such as the one below:
 
∫ Ln(x)/x2 dx
 
At a first glance, we have no idea how to integrate this function. We do, however, know how to integrate 1/x2 (which is our v' in this case), and we know how to differentiate Ln(x) (which is our u). So, using integration by parts:
 
∫ Ln(x)/x2 dx = ∫ Ln(x) · (1/x2) dx = (-1/x) · Ln(x) - ∫ (-1/x) · (1/x) d
-Ln(x)/x + ∫ (1/x2) dx = -Ln(x)/x - 1/x + C
 
(Not forgetting our constant of integration at the end!)

Answered by Alexander J. Maths tutor

2800 Views

See similar Maths IB tutors

Related Maths IB answers

All answers ▸

The velocity of a particle is given by the equation v= 4t+cos4t where t is the time in seconds and v is the velocity in m s ^-1. Find the time t when the particle is no longer accelerating for the interval 0≤t≤2.


What is a geometric sequence?


Consider the infinite geometric sequence 25 , 5 , 1 , 0.2 , ... (a) Find the common ratio. (b) Find (i) the 10th term; (ii) an expression for the nth term. (c) Find the sum of the infinite sequence.


Find the coordinates and determine the nature of the stationary points of curve y=(2/3)x^3+2x^2-6x+3


We're here to help

contact us iconContact usWhatsapp logoMessage us on Whatsapptelephone icon+44 (0) 203 773 6020
Facebook logoInstagram logoLinkedIn logo
Cookie Preferences