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sarihmanlapaz

  • 2 years ago

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

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  1. AravindG
    • 2 years ago
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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!!

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

  3. sarihmanlapaz
    • 2 years ago
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    what will i use in the 3 formula of cos2a?

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

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

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

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

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

  9. sarihmanlapaz
    • 2 years ago
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    so how will i substitute it to that equation?

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

  11. AravindG
    • 2 years ago
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    @sarihmanlapaz gt it?

  12. sarihmanlapaz
    • 2 years ago
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    got it! thanks :)

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

  14. hartnn
    • 2 years ago
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    what did u get ?

  15. sarihmanlapaz
    • 2 years ago
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    0.2435

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

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

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

  19. sarihmanlapaz
    • 2 years ago
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    its written in the question that the A is in quadrant 3

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

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

  22. sarihmanlapaz
    • 2 years ago
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    pls can you help me?

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

  24. sarihmanlapaz
    • 2 years ago
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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)

  25. hartnn
    • 2 years ago
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    u mean ....(1+cos A) ?

  26. sarihmanlapaz
    • 2 years ago
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    yes. yes :) i got the right answer 0.2425

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

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

  29. sarihmanlapaz
    • 2 years ago
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    there is the formula for half angle :)

  30. hartnn
    • 2 years ago
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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)}

  31. sarihmanlapaz
    • 2 years ago
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    yeah. thanks i got the right answer.

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