# Thread: Application of a Sum Formula

1. ## Application of a Sum Formula

"Write cos(arctan 1 + arccos x) as an algebraic expression." So, I understand that I am to use formula for cos(u + v) because it fits the expression, where u = arctan 1 and
v = arccos x. I also know that I must create to right triangles for each of the angles u and v, the reason being that the angles are likely to be different. What I do not understand is why, when the formula is used, we don't evaluate cos(arctan 1) as
cos(pi/4), which then equals 0. In the book, cos(arctan 1) is evaluated as
1/sqroot(2). Perhaps I do not understand inverse trigonometric functions as I thought I did.

2. ## Re: Application of a Sum Formula

Originally Posted by Bashyboy
"Write cos(arctan 1 + arccos x) as an algebraic expression." So, I understand that I am to use formula for cos(u + v) because it fits the expression, where u = arctan 1 and
v = arccos x. I also know that I must create to right triangles for each of the angles u and v, the reason being that the angles are likely to be different. What I do not understand is why, when the formula is used, we don't evaluate cos(arctan 1) as
cos(pi/4), which then equals 0. In the book, cos(arctan 1) is evaluated as
1/sqroot(2). Perhaps I do not understand inverse trigonometric functions as I thought I did.
pi/4 is on the unit circle and its cosine is not 0: $\displaystyle \cos \left(\frac{\pi}{4}\right) = \dfrac{\sqrt2}{2}$.

3. ## Re: Application of a Sum Formula

Originally Posted by Bashyboy
"Write cos(arctan 1 + arccos x) as an algebraic expression." So, I understand that I am to use formula for cos(u + v) because it fits the expression, where u = arctan 1 and
v = arccos x. I also know that I must create to right triangles for each of the angles u and v, the reason being that the angles are likely to be different. What I do not understand is why, when the formula is used, we don't evaluate cos(arctan 1) as cos(pi/4), which then equals 0. In the book, cos(arctan 1) is evaluated as 1/sqroot(2). Perhaps I do not understand inverse trigonometric functions as I thought I did.
This is standard fair: $\displaystyle \arctan (1) = \frac{\pi }{4}\;\& \,\cos \left( {\frac{\pi }{4}} \right) = \frac{{\sqrt 2 }}{2}$

4. ## Re: Application of a Sum Formula

You two are certainly right--and I am grateful for that. Sorry for the imprudent observation on my part.

5. ## Re: Application of a Sum Formula

Originally Posted by Bashyboy
"Write cos(arctan 1 + arccos x) as an algebraic expression." So, I understand that I am to use formula for cos(u + v) because it fits the expression, where u = arctan 1 and
v = arccos x. I also know that I must create to right triangles for each of the angles u and v, the reason being that the angles are likely to be different. What I do not understand is why, when the formula is used, we don't evaluate cos(arctan 1) as
cos(pi/4), which then equals 0. In the book, cos(arctan 1) is evaluated as
1/sqroot(2). Perhaps I do not understand inverse trigonometric functions as I thought I did.
An alternative is to note that

\displaystyle \displaystyle \begin{align*} \sin^2{X} + \cos^2{X} &\equiv 1 \\ \frac{\sin^2{X}}{\cos^2{X}} + \frac{\cos^2{X}}{\cos^2{X}} &\equiv \frac{1}{\cos^2{X}} \\ \tan^2{X} + 1 &\equiv \frac{1}{\cos^2{X}} \\ \frac{1}{\tan^2{X} + 1} &\equiv \cos^2{X} \\ \cos{X} &\equiv \sqrt{\frac{1}{\tan^2{X} + 1}} \end{align*}

So that means

\displaystyle \displaystyle \begin{align*} \cos{\left(\arctan{1}\right)} &= \sqrt{\frac{1}{\left[\tan{\left(\arctan{1}\right)}\right]^2 + 1}} \\ &= \sqrt{\frac{1}{1^2 + 1}} \\ &= \sqrt{\frac{1}{1 + 1}} \\ &= \sqrt{\frac{1}{2}} \\ &= \frac{\sqrt{1}}{\sqrt{2}} \\ &= \frac{1}{\sqrt{2}} \end{align*}