# Thread: Finding x in Trig

1. ## Finding x in Trig

Question:
Find sin(arccos(x))/cos(arcsin(x))

I am not sure if this helps to solve anything, and im not sure if this si correct or not.
=tan(arccos(x)/arcsin(x)
=tan(arccot(x))
=x
Thank you

2. ## Re: Finding x in Trig

Hint: use the identity $\arcsin(x)+\arccos(x)=\frac{\pi}{2}$...

3. ## Re: Finding x in Trig

Hello, DylanRenke!

$\text{Find: }\:\frac{\sin(\arccos x)}{\cos(\arcsin x )}$

Let: . $\begin{Bmatrix}\alpha &=& \arccos x & [1] \\ \beta &=& \arcsin x & [2] \end{Bmatrix}$

The problem becomes: . $\frac{\sin\alpha}{\cos\beta}$ .(a)

From [1]: / $\alpha \:=\:\arccos x \quad\Rightarrow\quad \cos\alpha \,=\,x$

We have: . $\cos\alpha \,=\,\frac{x}{1} \,=\,\frac{adj}{hyp}$

We have a right triangle with: $adj = x,\:hyp = 1.$

Pythagorus tells us that: . $opp \,=\,\sqrt{1-x^2}$

Hence: . $\sin\alpha \:=\:\frac{opp}{hyp} \:=\:\frac{\sqrt{1-x^2}}{1} \:=\:\sqrt{1-x^2}$ .(b)

From [2]: . $\beta \,=\,\arcsin x \quad\Rightarrow\quad \sin\beta \,=\,x$

We have: . $\sin\beta \,=\,\frac{x}{1} \,=\,\frac{opp}{hyp}$

We have a right triangle with: $opp = x.\:hyp \,=\,1.$

Pythagorus tells us that: . $adj \,=\,\sqrt{1-x^2}$

Hence: . $\cos\beta \:=\:\frac{adj}{hyp} \:=\:\frac{\sqrt{1-x^2}}{1} \:=\:\sqrt{1-x^2}}$ .(c)

Substitute (b) and (c) into (a): . $\frac{\sin\alpha}{\cos\beta} \;=\;\frac{\sqrt{1-x^2}}{\sqrt{1-x^2}} \;=\;1$

4. ## Re: Finding x in Trig

Originally Posted by DylanRenke
Question:
Find sin(arccos(x))/cos(arcsin(x))

I am not sure if this helps to solve anything, and im not sure if this si correct or not.
=tan(arccos(x)/arcsin(x)
=tan(arccot(x))
This is wrong. "arccos(x)/arcsin(x)" is NOT "arccot(x)". Just because a function has some property doesn't mean its inverse has that property!

=x
Thank you
Notice that $sin(u)= \pm\sqrt{1- cos^2(u)}$ and so $sin(arccos(x))= \pm\sqrt{1- cos^2(arccos(x)}= \pm\sqrt{1- x^2}$
However, cosine and sine are not "one to one" functions so they do NOT, strictly speaking, have inverses. In order to be able to talk about "arccos(x)" and "arcsin(x)" we hage to restrict cosine to 0 to $\pi$ and sine to $-\pi/2$ to [tex]\pi/2[/itex]. With those restrictions, sine and cosine are both postive and so both $cos(arcsin(x))$ and $sin(arccos(x))$ are equal to [tex]\sqrt{1- x^2}[tex].

5. ## Re: Finding x in Trig

If you use the hint I gave, you find:

$\frac{ \sin( \arccos(x))}{ \cos( \arcsin(x))}= \frac{ \sin( \arccos(x))}{ \cos\left( \frac{\pi}{2}-\arccos(x) \right)}= \frac{ \sin( \arccos(x))}{ \sin( \arccos(x))}=1$

6. ## Re: Finding x in Trig

Thank you everybody.