Question

URGENT PLEASE: given that sin 2pi/7 = 0.7818
evaluate TO 4 decimal places:
A) cos (3pi/14)
B) cos (11pi/14)

Answer 1

using equivalent algebraic expressions

Answer 2

pulling out trig cheat sheet...

Answer 3

ok lol...I will wait for you :D

Answer 4

i just need the concept of how you go about doing these kinda questions...

Answer 5

actually I have to go
do what I would do: look at this sheet for some relevant formulae
http://tutorial.math.lamar.edu/pdf/Trig_Cheat_Sheet.pdf

Answer 6

make use of things like half-angle formulas, things like \(\sin(\frac\pi2-\theta)=\cos\theta\)

Answer 7

looks like you almost have \(\large \frac{2\pi}7=\frac{4\pi}{14}=\frac{\pi}{14}+\frac{3\pi}{14}\) so maybe some of those addition/subtraction formulas will come in handy

Answer 8

ok I get how I can manipulate the term in the middle of what is given for example in this care the 2pi/7
but how do I go from there to finding the cos 3pi/14 term

Answer 9

like I can say:

\[\sin(\pi/14 +3\pi/14) = \sin(\pi/14)\cos(3\pi/14) +\cos(\pi/14)\sin(3\pi/14) \]

but I don't know if that will help

Answer 10

you're basically trying the same type of stuff that I would, so continue looking at double-angle formulas to get that pi/14 argument to be a pi/7 argument
I'm sorry I have to go, sorry to bail this time :(
good luck!

Answer 11

lol its iight thanks a lot man, I like the cheat sheet btw

Answer 12

k wait i gotta read the question

Answer 13

haha i dont know what to do lol

Answer 14

its i got this one:
so the solution is,
for A) you say since \[\sin(\theta)=\cos({\pi/2} - \theta)\]

then
sin(4pi/14)= (cos pi/2 - 4pi/14) = cos (3pi/14) = 0.7818

for B) you use sin(theta) = -cos(pi/2+theta) and answer is= -0.7818

Answer 15

ohhh i see

Answer 16

haha i never done this stuff lol

Answer 17

it looks fun

Answer 18

yeh it is fun... I have done it 2 years ago, I cant remember anything now :S

Answer 19

haha i have never done this but like i wanna learn it but i am dead at this point. So i cant really think

Answer 20

i am actually gonna save that trig sheet on my comp. It is a good thing to have

Answer 21

yeh it is very good i love it :D

Answer 22

so who r u tutoring?

Answer 23

some guy I don't know

Answer 24

lol in ur neighbourhood?

Answer 25

how do you tutor? give me tips and stuff

Answer 26

kinda yeh

Answer 27

well i am tutoring a grade 8er so its diff

Answer 28

and do you ask them to tell you what they want you to teach them the day before?

Answer 29

umm i dont but if its hs math then yes u do

Answer 30

like u need to prepare

Answer 31

but like u explain the topic and then u do one example together and then u let the guy do another one on his own

Answer 32

and if he needs help then help him out on the way but try not to give away the answer

Answer 33

ok cool i get that, now lets say you get across a question that you don't know lol

Answer 34

what do you do in that case?

Answer 35

ummmm i just say i dont know but u shld say like i dont know this let me work this out and get back to u tom. ok like even if u dont tutor him th enext day like email him the answer

Answer 36

and if this guy is academically low then sometimes explaining the logic of a concept can confuse him so u just explain him the rules. It all depends on the student

Answer 37

i see.. yeh yeh i get what you mean

Answer 38

ok samjordan, amma go sleep am so tired

Answer 39

gd night.

Answer 40

ok bye