algebra!!

http://books.google.com/books?id=K6WrCgekj3MC&lpg=PA189&ots=WYlcwpP7Qo&dq=cual%20es%20la%20equivalencia%20de%201%2Fsenx%20%20%20(trigonometria)&hl=es&pg=PA191#v=onepage&q&f=false