sec²x - sec²x·sin²x

To put that in your calculator

(1/cos(x))²-(1/cos(x))²(sin(x))²

To graph that on a TI-83 or 84, don't press GRAPH, 
but instead press ZOOM, then press 7


Factor out sec²x

sec²x(1 - sin²x)

Use the identity: cos²x + sin²x = 1 and solve for cos²x
                          cos²x = 1 - sin²x
So replace 1 - sin²x by cos²x

sec²x(cos²x)

Then replace sec²x by 

(cos²x)· =

 =

           1 

The constant 1.  However, sec²x - sec²x·sin²x is not defined
as 1 or as any number at all when x = any odd multiple of 90°
if you're using degrees or  if you're using radians.                

Edwin