Question

use half angle identity for :)

cos 3pi/8

Answer 1

http://www.purplemath.com/modules/idents.htm

Use the ID for cos(x/2)

Write \(\Large \dfrac{3\pi}{8}\) as \(\Large \dfrac{1}{2}\cdot x \) , so x=? (It will be one of your unit circle angle measures, so you know the cos(x) value that you have to use in the identity)

Then just plug & chug in that identity! You know this is in the first quadrant, so you know the sign of the result.

Answer 2

I don;t get the part about 1/2 * x thats where I'm getting confuseed

Answer 3

@DebbieG

Answer 4

is it 3pi over 4 ?

Answer 5

lol... this is similar to the apples. :)

You need to figure out an x, so that you can write the angle as
\(\Large \dfrac{1}{2}\cdot x\)

In other words, you need:

\(\Large \dfrac{3\pi}{8}=\dfrac{1}{2}\left(???\right)\)

Answer 6

apples :D

Answer 7

Right!

\(\Large \dfrac{3\pi}{8}=\dfrac{1}{2}\left(\dfrac{3\pi}{4}\right)\)

yes, apples... :)

Answer 8

Thank you thank you thank you!

so in half angle Identity should be , cos 3pi/2
______ ?
2

Answer 9

Not quite. You want:

\(\Large \cos\dfrac{3\pi}{8}=\cos\dfrac{x}{2}=\cos\dfrac{3\pi/4}{2}\)

Remember, you figured out above that your \(x=\dfrac{3\pi}{4}\)

Answer 10

so we say 3pi/8 over 2 is equal to 3pi / 4 ?

Answer 11

apples..

Answer 12

Because, \(\Large \dfrac{3\pi}{8}=\dfrac{1}{2}\cdot \dfrac{3\pi}{4}\)

Answer 13

No, you have it backwards.

NOT: "3pi/8 over 2 is equal to 3pi / 4 ? "

Answer 14

But: 3pi/8 is equal to (3pi / 4) over 2

Answer 15

I see! and we get the half angle identity then?

Answer 16

Remember, dividing by 2 is EXACTLY THE SAME as multiplying by 1/2, so it's easier to look at these kinds of expressions as:

\(\Large \dfrac{3\pi}{8}=\dfrac{1}{2}\cdot \dfrac{3\pi}{4}\)

Yes, use the ID for cos(x/2), with \(\Large x= \dfrac{3\pi}{4}\)

Answer 17

having cos 3pi / 4 over 2 = (identity formula )

Answer 18

Thanks! again @DebbieG !!!!

Answer 19

You're welcome. :)

Answer 20

:)