anonymous
  • anonymous
If cosA= -15/17 and A is in quadrant III, find cos 1/2A
Mathematics
schrodinger
  • schrodinger
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AravindG
  • AravindG
ya you have the value of cos A try to use the formula of cos 2A to get to the answer!!
AravindG
  • AravindG
@sarihmanlapaz did you try that?
anonymous
  • anonymous
what will i use in the 3 formula of cos2a?

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AravindG
  • AravindG
just look at the equations and look at what we are trying to get the answer in terms of
AravindG
  • AravindG
we want cos A/2 isnt it?
AravindG
  • AravindG
and we have cos A , now you tell me which equation to use!!
anonymous
  • anonymous
i think cos^2 - 1
AravindG
  • AravindG
\[2\cos^2 x-1\]
anonymous
  • anonymous
so how will i substitute it to that equation?
hartnn
  • hartnn
cosA=2cos^2(A/2)-1 find cos(A/2) from here.
AravindG
  • AravindG
anonymous
  • anonymous
got it! thanks :)
anonymous
  • anonymous
but its none of the choices. >.< how can i solve it?
hartnn
  • hartnn
what did u get ?
anonymous
  • anonymous
0.2435
hartnn
  • hartnn
i m getting -0.2435, is that the option ?
anonymous
  • anonymous
oh no. haha my answer 0.2435 is in the options. i did'nt notice it.
hartnn
  • hartnn
but A/2 will be in 2nd quadrant where cos is negative. so we put minus -0.2435
anonymous
  • anonymous
its written in the question that the A is in quadrant 3
hartnn
  • hartnn
yes, A in 3rd,A/2 in 2nd. is -0.2435 in options?
anonymous
  • anonymous
the options are. A).0.24054 B).0.24125 C) 0.24254 D) 0.24354
anonymous
  • anonymous
pls can you help me?
hartnn
  • hartnn
since all options are positive, i guess A/2 goes in 1st quadrant...then 0.2435 is correct.
anonymous
  • anonymous
no. its wrong my friend. i just check the half angle formula. haha there is a formula for cosA/2 = squareroot of 1/2(1+A)
hartnn
  • hartnn
u mean ....(1+cos A) ?
anonymous
  • anonymous
yes. yes :) i got the right answer 0.2425
hartnn
  • hartnn
how? the formula we gave and what u wrote are exactly the same...
anonymous
  • anonymous
http://www.themathpage.com/atrig/trigonometric-identities.htm
anonymous
  • anonymous
there is the formula for half angle :)
hartnn
  • hartnn
so using that u got different answer? we gave u that formula only..... cosA=2cos^2(A/2)-1 so cos(A/2)=sqrt{0.5*(1+cos A)}
anonymous
  • anonymous
yeah. thanks i got the right answer.

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