### Author Topic: Lec5101 Question Mock Quiz 1  (Read 1438 times)

#### yingxuanwang

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##### Lec5101 Question Mock Quiz 1
« on: September 19, 2020, 09:11:39 PM »
Hi, I have a question when I solve the general solution of ODE.
When I calculate the integral, should I add the absolute value to the result? (see the picture)
I am confused about this because I think the constant "C" can cancel the absolute value there.
Also, can anyone help me to check if my answer totally correct or anywhere I can improve?
Look forward to a kind reply.
Thanks a lot!

#### Lingfei Li

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##### Re: Lec5101 Question Mock Quiz 1
« Reply #1 on: September 20, 2020, 12:17:47 PM »
For your first question, the integral should be the absolute value.
However, when you are doing such problems, I believe that you can just assume everything in the absolute value symbol is positive because you already had a constant C (except for some cases that have a restriction on the variables). Also, I think your answer is correct and well-written.
Correct me if I'm wrong!

#### yingxuanwang

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##### Re: Lec5101 Question Mock Quiz 1
« Reply #2 on: September 20, 2020, 01:23:34 PM »
I am still a little bit confused. So for this question, the result of the integral does not need to contain absolute value right? Or I need to write it in two lines: 1st line is with the absolute value and constant C1, 2nd line is without the absolute value and constant C?
« Last Edit: September 20, 2020, 01:27:04 PM by yingxuanwang »

#### Victor Ivrii

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##### Re: Lec5101 Question Mock Quiz 1
« Reply #3 on: September 20, 2020, 03:14:04 PM »
Read W2L1 handout. THere is an explanation

#### Lingfei Li

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##### Re: Lec5101 Question Mock Quiz 1
« Reply #4 on: September 20, 2020, 09:09:14 PM »
The integral should have the absolute value,  but for this question, your result don't need it. In your process, you can remove the absolute value because constant C can be positive, negative or 0. You can read the 8th page of the slide to see details.