Question

How do you find exact value?

Cos (- 3pi/4)

Answer 1

hint:
we can write this:
\[\begin{gathered}
\cos \left( { - \frac{{3\pi }}{4}} \right) = \cos \left( {\frac{\pi }{4} - \pi } \right) = \hfill \\
\hfill \\
= \cos \left( {\frac{\pi }{4}} \right)\cos \left( \pi \right) + \sin \left( {\frac{\pi }{4}} \right)\sin \left( \pi \right) = ...? \hfill \\
\end{gathered} \]

Answer 2

What happened to the three?

Answer 3

we have:
\[\frac{\pi }{4} - \pi = - \frac{{3\pi }}{4}\]

Answer 4

cos (-theta) = cos theta

Answer 5

cos(-3pi/4)= cos (3pi/4)

Answer 6

How does that end up as - 1/√ 2?

Answer 7

cos(3*180/4)= cos(3*45)
= cos135

Answer 8

cos(90+45) = -sin45

Answer 9

=-1/square root of 2

Answer 10

Wow. That was way simpler than I made it out to be. Thanks a bunch!