Trig question -sin pi/4 , give the exact value?

[Neopets]

Homework Statement

-sin pi/4 , give the exact value?
-1/root 2 is the answer according to the book? How in the world do they get that result. What do you to make that happen?

Homework Equations

The Attempt at a Solution

Also, is this how to do the problem? Find pi/4 on the unit circle, it's root 2 / 2, that doesn't seem to get the right answer even though that's was supposed to be the method?

 

[QuarkCharmer]

$\frac{1}{\sqrt{2}}$ is the same as $\frac{\sqrt{2}}{2}$, simply "rationalize" it by multiplying the top and bottom by $\sqrt{2}$

Basically you have the right idea, you can think of sin as the y-coordinate.
Remember, the problem is "negative" sin pi/4.

 

[bacon]

How in the world do they get that result. What do you to make that happen?

Take a look at the Pythagorean Theorem.

http://en.wikipedia.org/wiki/Pythagorean_theorem

 

[2milehi]

It is sloppy to leave a radical in the denominator.

 

[Mentallic]

It is sloppy to leave a radical in the denominator.

Not in all cases
I don't think the teacher would worry so much about it when it comes to trig.