# Thread: Finding exact value of trigonometric expression

1. ## Finding exact value of trigonometric expression

Hello Everyone,

I'm having a little bit of trouble with the following problem.

I need to find $cos(v - u)$ using $sin\ u = \frac{7}{25}, cos\ v = -\frac{3}{5}$

I tried using those values to find the missing functions
$sin\ u = \frac{7}{25}, cos\ u = \frac{24}{25} \ and \ cos\ v = -\frac{3}{5}, sin\ v = \frac{4}{5}$

Then i used the following formula ... $cos(v - u) = cos\ v \cdot cos\ u + sin\ v \cdot sin\ u$ Replaced the numbers

$-\frac{3}{5} \cdot \frac{24}{25} + \frac{4}{5} \cdot \frac{7}{25}$
=

$-\frac{44}{175}$

2. ## Re: Finding exact value of trigonometric expression

Originally Posted by vaironxxrd
Hello Everyone,

I'm having a little bit of trouble with the following problem.

I need to find $cos(v - u)$ using $sin\ u = \frac{7}{25}, cos\ v = -\frac{3}{5}$

I tried using those values to find the missing functions
$sin\ u = \frac{7}{25}, cos\ u = \frac{24}{25} \ and \ cos\ v = -\frac{3}{5}, sin\ v = \frac{4}{5}$

Then i used the following formula ... $cos(v - u) = cos\ v \cdot cos\ u + sin\ v \cdot sin\ u$ Replaced the numbers

$-\frac{3}{5} \cdot \frac{24}{25} + \frac{4}{5} \cdot \frac{7}{25}$
=

$-\frac{44}{175}$
You can't get a unique answer unless you're told something about u and v, i.e. which quadrants they lie in...

3. ## Re: Finding exact value of trigonometric expression

Originally Posted by Prove It
You can't get a unique answer unless you're told something about u and v, i.e. which quadrants they lie in...
Both u and v lie in Quadrant 2.

In that case.

$sin\ u = \frac{7}{25}, \ x = -24, \ y = 7 \ r = 25$ making $cos\ u = -\frac{24}{25}$
$cos\ v = -\frac{3}{5}. \ x = - 3, \ y = 4, \ r = 5$ making $sin v = \frac{4}{5}$

Then $cos(v - u) = cos\ v \cdot cos\ u + sin\ v \cdot sin\ u$ Replaced the numbers

$-\frac{3}{5} \cdot -\frac{24}{25} + \frac{4}{5} \cdot \frac{7}{25}$ = $\frac{72}{125} + \frac{28}{125}$
= $\frac{4}{5}$