Hello Jamal

How are you doing?

As already explianed to you , for solving these tyoe of problems just convert the inside arc function into same as that of outside function 

 for example

20) sec ( Arc cos 2/5 )

 convert the arc cos 2/5 into sec function

its equal to arc sec 5/2

 just put its value

 sec ( arc sec 5/2)

= 5/2

Hope this helps