# Trig exact values

• Apr 22nd 2008, 08:39 PM
Ericonda
Trig exact values
Find the exact value of sin 195° given sin 195°=sin (225°-30°)

Basically what I need to know is how sin 225° = -sin 45°....how do I know that sin 225° is equal to -sin 45°?
• Apr 22nd 2008, 08:43 PM
Mathstud28
Quote:

Originally Posted by Ericonda
Find the exact value of sin 195° given sin 195°=sin (225°-30°)

Basically what I need to know is how sin 225° = -sin 45°....how do I know that sin 225° is equal to -sin 45°?

$sin(225-30)=\sin(225)cos(30)-cos(225)sin(30)=\frac{-\sqrt{2}}{2}\frac{\sqrt{3}}{2}-\frac{-\sqrt{2}}{2}\frac{1}{2}$
you can multiply
• Apr 23rd 2008, 02:40 AM
Moo
Quote:

Originally Posted by Ericonda
how do I know that sin 225° is equal to -sin 45°?

Hello,

Because 225°=180+45

And if you look at a unit circle, sin(180+x)=-sin(x) ;)