# Math Help - evaluating the following.

1. ## evaluating the following.

evaluating
$

sin(\frac{-9\pi}{4})

$

just wondering how they got

$

\frac{-1}{\sqrt{2}}

$

i got

$

\frac{-\sqrt{2}}{2}

$

but was wrong

2. Try this

$\bigg(\frac{-\sqrt{2}}{2}\bigg)\bigg(\frac{\sqrt{2}}{\sqrt{2}}\ bigg)$

$\frac{-2}{2\sqrt{2}}$

$\frac{-1}{\sqrt{2}}$

3. I got 2 root 2 as well..isn't this in the 1st quadarant...hence Sin = a positive...

4. Its in the 4th quadarant

5. Originally Posted by justinwager
I got 2 root 2 as well..isn't this in the 1st quadarant...hence Sin = a positive...
yeah thats what i thought.. its wierd. dont know how they got the -1 and root 2 on the denom

6. Originally Posted by 11rdc11
yeah coz u go backyards , but it still shud be what i wrote..

7. Originally Posted by 11rdc11
Try this

$\bigg(\frac{-\sqrt{2}}{2}\bigg)\bigg(\frac{\sqrt{2}}{\sqrt{2}}\ bigg)$

$\frac{-2}{2\sqrt{2}}$

$\frac{-1}{\sqrt{2}}$
why did u multiply it by 1 ? sqrt 2 / sqrt 2

8. To make it look like the answer provided. It doesn't change the answer since what I did to the numerator I also did to the denominator so it valid

$\frac{-\sqrt{2}}{2} = \frac{-1}{\sqrt{2}}
$