# Inequalities & A Definite Integral

• May 12th 2013, 11:09 AM
zachd77
Inequalities & A Definite Integral
I don't understand the first step of the following proof (I was given the first step and I completed it fine, but it is the first step that I do not understand).

Attachment 28346

First step:

Attachment 28347

The rest of the proof was straightforward. From the first step, this next part comes from the fact that the resulting integral from the first step is less than or equal to:

Attachment 28348
Anyways, I just need help understanding why the first step is true. Thanks for any help!!!
• May 12th 2013, 11:32 AM
Plato
Re: Inequalities & A Definite Integral
Quote:

Originally Posted by zachd77
I don't understand the first step of the following proof (I was given the first step and I completed it fine, but it is the first step that I do not understand).
Attachment 28346

First step:
Attachment 28347

It is a theorem in the theory of integrals:

If $f$ is integrable on $[a,b]$ then $|f|$ is integrable on $[a,b]$ and

$\left| {\int_a^b f } \right| \leqslant \int_a^b {\left| f \right|}~.$

Is that the question?
• May 12th 2013, 11:59 AM
zachd77
Re: Inequalities & A Definite Integral
Yep! That is what I am looking for; I have not seen that property of integrals before.

Thanks for the help!