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sarihmanlapaz

If cosA= -15/17 and A is in quadrant III, find cos 1/2A

  • one year ago
  • one year ago

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  1. AravindG
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    ya you have the value of cos A try to use the formula of cos 2A to get to the answer!!

    • one year ago
  2. AravindG
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    @sarihmanlapaz did you try that?

    • one year ago
  3. sarihmanlapaz
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    what will i use in the 3 formula of cos2a?

    • one year ago
  4. AravindG
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    just look at the equations and look at what we are trying to get the answer in terms of

    • one year ago
  5. AravindG
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    we want cos A/2 isnt it?

    • one year ago
  6. AravindG
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    and we have cos A , now you tell me which equation to use!!

    • one year ago
  7. sarihmanlapaz
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    i think cos^2 - 1

    • one year ago
  8. AravindG
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    \[2\cos^2 x-1\]

    • one year ago
  9. sarihmanlapaz
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    so how will i substitute it to that equation?

    • one year ago
  10. hartnn
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    cosA=2cos^2(A/2)-1 find cos(A/2) from here.

    • one year ago
  11. AravindG
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    @sarihmanlapaz gt it?

    • one year ago
  12. sarihmanlapaz
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    got it! thanks :)

    • one year ago
  13. sarihmanlapaz
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    but its none of the choices. >.< how can i solve it?

    • one year ago
  14. hartnn
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    what did u get ?

    • one year ago
  15. sarihmanlapaz
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    0.2435

    • one year ago
  16. hartnn
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    i m getting -0.2435, is that the option ?

    • one year ago
  17. sarihmanlapaz
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    oh no. haha my answer 0.2435 is in the options. i did'nt notice it.

    • one year ago
  18. hartnn
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    but A/2 will be in 2nd quadrant where cos is negative. so we put minus -0.2435

    • one year ago
  19. sarihmanlapaz
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    its written in the question that the A is in quadrant 3

    • one year ago
  20. hartnn
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    yes, A in 3rd,A/2 in 2nd. is -0.2435 in options?

    • one year ago
  21. sarihmanlapaz
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    the options are. A).0.24054 B).0.24125 C) 0.24254 D) 0.24354

    • one year ago
  22. sarihmanlapaz
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    pls can you help me?

    • one year ago
  23. hartnn
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    since all options are positive, i guess A/2 goes in 1st quadrant...then 0.2435 is correct.

    • one year ago
  24. sarihmanlapaz
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    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)

    • one year ago
  25. hartnn
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    u mean ....(1+cos A) ?

    • one year ago
  26. sarihmanlapaz
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    yes. yes :) i got the right answer 0.2425

    • one year ago
  27. hartnn
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    how? the formula we gave and what u wrote are exactly the same...

    • one year ago
  28. sarihmanlapaz
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    http://www.themathpage.com/atrig/trigonometric-identities.htm

    • one year ago
  29. sarihmanlapaz
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    there is the formula for half angle :)

    • one year ago
  30. hartnn
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    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)}

    • one year ago
  31. sarihmanlapaz
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    yeah. thanks i got the right answer.

    • one year ago
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