Question

Use the appropriate sum or difference identity to find the exact value of cos (7pi/12)

Answer 1

-.26 ?

Answer 2

cos (pi/3 + pi/4) = cos pi/3 cos pi/4 - sin pi/3 sinn pi/4
= 1/2 * 1 / sqrt2 - sqr3/2 * 1/sqrt2
= 1 / 2sqrt2 - sqrt3 / 2sqrt2
= (1 - sqrt3) / 2 sqrt2

Answer 3

- if my arithmetic is correct
- ill check it by calculator

Answer 4

(1 - sqrt3) / 2 sqrt2

isnt an option

Answer 5

yes - its correct

Answer 6

hey if you click the attachment and look at #23, you will see that i dont have that option

Answer 7

ok

Answer 8

right - i think its there but written with a rational denominator

multipl top and bottom by sqrt2:

1 - sqrt3 * sqrt2 = sqrt2 - sqrt6
------- ---- -----------
2 sqrt 2 * sqrt2 = 4

thats one of the options right?

Answer 9

its always tidier to rationalise the denom.

Answer 10

yea

Answer 11

are you a math teacher

Answer 12

i used to be - retired now - thats my cat and grandaughter on my pic

Answer 13

oh ok. what part of UK do you live?

Answer 14

swansea

Answer 15

kool. I have really appreciate all the help you given me

Answer 16

ur welcome r u from uk?

Answer 17

nah I live in the states, but I'm somewhat familiar with England. I though thats where you were from

Answer 18

smart of you - by the way i write i suppose - i'm from wales - west part of uk - great place but it rains a lot. swansea is the home of dylan thomas - a poet who was popular in the states

Answer 19

yeah I did quick research on swansea after you told me

Answer 20

well talk to you again - time for me to help my wife wash up!!

Answer 21

ok take care