No one sings about math as well as Danny Kaye. A Canajun writer for McLeans, Jaime J. Weinman mentions his prowess at explaining the Pythagoreum theorum. (Perhaps it would have been wiser of the Scarecrow to have listened to this song before showing off his new brains.)
My personal favourite of his is from Hans Christian Anderson where he sings about the Inchworm. (Pictured is Charles Aznavour who sang it on the Muppet Show prior to Danny. Though Big Bird beat them both to it on Sesame Street.)
Click away for more Math in the Movies. Yet I'd be remiss if I didn't mention Tom Lehrer's genius for mathematical tunes. His best would be New Math, explaining how to subtract 173 from 342 in base 10 and in base 8.
Though it's not about math, it is one of my favourite tunes of his where he can be found singing periodically.