Question

by USING HALF ANGLE IDENTITIES,find the exact values of cos 7 pi/8

Answer 1

cos 7 pi/8= Cos ( pie - pie/ 8) = -cos pie/8

Answer 2

hiw did u get -cos?

Answer 3

let

2x = pie/8

x = pie/4

cos2x = 2cos^2x - 1

- cos 2x = 1 - 2 cos^2 x

Answer 4

Substitute the values

Answer 5

(pie - pie/8) is in 2 nd quadrant

Answer 6

that's how u get -cos?

Answer 7

Yes

Answer 8

but the method that u did is not how i am suppose to do i have to use the half angle identities can u show me how to do it with that identitis?

Answer 9

PLZA

Answer 10

im not more like to pi, actually i have to convert pi radian to degree,
7 pi/8= 157.5 deg = 315/2 deg :)

Answer 11

after that use the half angles formula :
cos(x/2) = sqrt[(1+cos(x))/2]
here x=315, try it ...

Answer 12

yeah that works but just tell show me how get right answer by using that identitie

Answer 13

use the formula above ^
cos(315/2) = sqrt[(1+cos(315))/2]
cos157.5 = sqrt[(1+1/2*sqrt(2))/2] = ... dont forget simplify :)

Answer 14

k the answer my teacher got is