tanx/cscx + cosx = secx
tanx/cscx + cosxcscx/cscx = secx
(sinx/cosx + cosx/sinx )cscx = secx
sin^2x + cos^2x
-------------------------------------
sinxcosx
---------------------------------------- = secx ; sin^2x + cos^2x = 1
1/sinx
1/cosx = secx
secx = secx