[tex]\bf 18)\\\\
csc\left(-\frac{\pi }{4} \right)=\stackrel{some~value}{y}\qquad therefore\qquad csc^{-1}(y)=-\frac{\pi }{4}
\\\\\\
19)\\\\
sec^{-1}(\sqrt{3})=\stackrel{some~angle}{\theta }\qquad therefore\qquad sec(\theta )=\sqrt{3}[/tex]
you can think of such combinations this way like in this example,
sin(90°) = 1, well, then what is sin⁻¹(1)?
well, sin⁻¹(1) is really asking "what is the angle or angles whose sine is 1?"
well, we already know that sin(90°) is 1 thus sin⁻¹( sin(90°) ) = 90°.
