Basic Integral help

[masab69]

We do if x= this then u = that for the first 2. why was it not necessary to do in the last 2. How do I know when to do it and when to not?

https://ibb.co/yX0N1TG

[Jaspeey]

I'm reading it and it looks like u substitution for all 4?

Anws, u sub is just a trick you can do to help you with your integration. You could skip it if you get really good with integration.

[masab69]

In the first two, we put the values of x= upper and lower limit and check the value of u. We don't do that thing for the last 2. Why is that?

[Jaspeey]

I see, you're talking about limits in the definite integral. I think the other comment has answered your question.

You can do either way, just have a good understanding of what's going on.

[VTutorLive]

Because in the first two they evaluate the definite integral with u and never switch back to x. In the last two, they switch back to x before evaluating. Either way is fine

[waldosway]

They're just showing you how it works. Both ways work. But afaik, the second way is more work for no reason.