Wednesday, June 8, 2011

girl math: bsb harmonic range

yesterday i was taking a trip down memory lane and watching backstreet boys music videos on youtube.  one of my favorites:



in mathematics, a harmonic range is the distance between four harmonic conjugates. 

let's say our backstreet boys are forming collinear points by standing on a line:


brian and nick are harmonic conjugates with respect to a.j. and howie if: 

|brian * a.j.|
=
|brian * howie|
|a.j.* nick|
|howie * nick|


further, a.j. and howie are harmonic conjugates with respect to brian and nick. 

in short, our boys divide the line in the same ratio.  line brian-nick is divided harmonically at a.j. (internal ratio) and at howie (external ratio).  if | brian * howie | = 1, then:

|brian * a.j.|
=
a ( 1 - a )
|brian * nick|
=
2a
1 + a
a + 1


all four boys form a harmonic range within our harmonic line segment! 

but what if kevin wants to rejoin the group?  no problem, he can stand as the midpoint between brian and nick:

then:

kevin * nick^2 = (kevin * a.j.) (kevin * howie) 

who doesn't love a good 1998 bsb harmony?