I'm reading:
http://www.feynmanlectures.caltech.e...3.html#Ch13-S4
1. In the link it says:
##2\pi\rho d\rho## is the area of the ring of the radius ##\rho## and width ##d\rho##, if ##d\rho \ll \rho##.
Why is this true??
2. A bit further down in the text it says:
Since ##r^2 = \rho^2 + a^2##, ##\rho\,d\rho = r\,dr##.
How do you get this result??
3. Even further down, after "But we see that", it says:
##r^2 = ... = a^2 + R^2 - 2Rx## Then: ##2r\,dr=-2R\,dx##
How do you get that?
It would be great if you answered my questions and if you recommended me some book or link that shows how to get these mathematical results.
http://www.feynmanlectures.caltech.e...3.html#Ch13-S4
1. In the link it says:
##2\pi\rho d\rho## is the area of the ring of the radius ##\rho## and width ##d\rho##, if ##d\rho \ll \rho##.
Why is this true??
2. A bit further down in the text it says:
Since ##r^2 = \rho^2 + a^2##, ##\rho\,d\rho = r\,dr##.
How do you get this result??
3. Even further down, after "But we see that", it says:
##r^2 = ... = a^2 + R^2 - 2Rx## Then: ##2r\,dr=-2R\,dx##
How do you get that?
It would be great if you answered my questions and if you recommended me some book or link that shows how to get these mathematical results.
