secx+sinx tanx=cosx are you sure it is a +? it doesn't work if it is.
if it is a - then it works
secx - sinx tanx = cosx
1 - sinx*sinx = cosx
------- -----
cosx cosx
1 - (sinx)^2
--------------- = cosx
cosx
(cosx)^2 = cosx
-----------
cosx
cosx = cosx proved