Which equation should be used to graph y=sec^-1(x) on a calculator ?

y=1/sec(x)
y=1/cos^-1(x)
y=sec(1/x)
y=cos^-1(1/x)

Arcsecant or arcsec or sec⁻¹ is an inverse trigonometric function. And many calculators don't have a key for them because they can be calculated with basic trigonometric functions like so:

sec⁻¹(x)=cos⁻¹(1/x). 

So the correct answer is the fourth one: y=cos⁻¹(1/x).