# Theta in converting sec to cos

## Homework Statement

I saw in my calculus book that something along the lines of arcsec $\frac{4}{\sqrt{pi}}$ = x was converted to arccos $\frac{\sqrt{pi}}{4}$ = x. I understand that sec and cos are reciprocals, but I don't see why has to be flipped as well.

cos (θ), sec (θ)

## The Attempt at a Solution

I began to think of the graphs and such, but I just can't think of why this works. I don't normally mess with the inverse trig functions and was hoping someone could point out what I'm missing.

Thank you.

Mark44
Mentor

## Homework Statement

I saw in my calculus book that something along the lines of arcsec $\frac{4}{\sqrt{pi}}$ = x was converted to arccos $\frac{\sqrt{pi}}{4}$ = x. I understand that sec and cos are reciprocals, but I don't see why has to be flipped as well.

cos (θ), sec (θ)

## The Attempt at a Solution

I began to think of the graphs and such, but I just can't think of why this works. I don't normally mess with the inverse trig functions and was hoping someone could point out what I'm missing.

Thank you.
Let x = $sec^{-1}\frac{4}{\sqrt{\pi}}$
Then sec(x) = $\frac{4}{\sqrt{\pi}}$
So cos(x) = $\frac{\sqrt{\pi}}{4}$
Which means that x = cos-1$\frac{\sqrt{\pi}}{4}$

It should be understood that there are domain restrictions on x.