Source Page

http://www.managementhelp.org/prsn_prd/prob_slv.htm

http://office.microsoft.com/en-us/onenote/HA101686341033.aspx

http://en.wikipedia.org/wiki/Image:Circle-trig6.svg [diagram]

http://www.ies.co.jp/math/java/trig/sixtrigfn/sixtrigfn.html