Question

when do we use u substation when we are integrating ?

Answer 1

You use the u sub to eliminate the x variable in order for the anti derivative becomes easily identifiable.

This site shows a list of problems that requires u sub
https://www.math.ucdavis.edu/~kouba/CalcTwoDIRECTORY/usubdirectory/USubstitution.html

Answer 2

for the invs trig and other functions you use u-sub when the derivative of `u` is already in the function

Answer 3

We're not out to eliminate variables here. Rather, we use substitutions to SIMPLIFY the integration. True, you may temporarily eliminate the variable x in favor of the new variable u, but in the end you'll likely have to revert to variable x.

Toss out a couple of the harder integration problems you're working on now. Then we could discuss how to determine whether substitution is a good method for a given problem and how to perform that subs.

Answer 4

okie ty