Question

in this derivation how and why x changed into r ​

Answer 1

Answer:

as x has a limit of distance r1 to r2 and there's a rule in intregation that the digits which is in brackets it will be written as upper limit - lower limit ❤

Answer 2

Answer:

The limit is applied to x and the value of x is shown by r.The upper limit is r2 and lower limit is r1

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