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

- anonymous

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- anonymous

using equivalent algebraic expressions

- TuringTest

pulling out trig cheat sheet...

- anonymous

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

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## More answers

- anonymous

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

- TuringTest

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

- TuringTest

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

- TuringTest

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

- anonymous

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

- anonymous

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

- TuringTest

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!

- anonymous

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

- anonymous

k wait i gotta read the question

- anonymous

haha i dont know what to do lol

- anonymous

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

- anonymous

ohhh i see

- anonymous

haha i never done this stuff lol

- anonymous

it looks fun

- anonymous

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

- anonymous

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

- anonymous

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

- anonymous

yeh it is very good i love it :D

- anonymous

so who r u tutoring?

- anonymous

some guy I don't know

- anonymous

lol in ur neighbourhood?

- anonymous

how do you tutor? give me tips and stuff

- anonymous

kinda yeh

- anonymous

well i am tutoring a grade 8er so its diff

- anonymous

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

- anonymous

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

- anonymous

like u need to prepare

- anonymous

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

- anonymous

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

- anonymous

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

- anonymous

what do you do in that case?

- anonymous

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

- anonymous

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

- anonymous

i see.. yeh yeh i get what you mean

- anonymous

ok samjordan, amma go sleep am so tired

- anonymous

gd night.

- anonymous

ok bye

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