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anonymous

  • 5 years ago

given sinx=-1/4 and 180<x<270, determine the exact value of cos2x

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  1. amistre64
    • 5 years ago
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    lets find the cos of x..... think we can?

  2. amistre64
    • 5 years ago
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    cos(2x) = cos(x)cos(x) - sin(x)sin(x)

  3. amistre64
    • 5 years ago
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    sin^2(x) + cos^2(x) = 1 cos^2(x) = 1 - (-1/4)^2 cos^2(x) = 1 -1/16 cos^2(x) = 15/16 cos(x) = sqrt(15)/4

  4. amistre64
    • 5 years ago
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    cos(2x) = sqrt(15)sqrt(15) -1(-1) ------------- - -------- 4(4) 4(4)

  5. amistre64
    • 5 years ago
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    cos(2x) = 15 - 1 ------ 16 cos(2x) = 14/16 = 7/8

  6. anonymous
    • 5 years ago
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    oh my gosh, that makes total sense now! i just have trouble picking what identity or formula to use.

  7. amistre64
    • 5 years ago
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    trig is 90% memorizing

  8. anonymous
    • 5 years ago
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    yeahh, tell me about it. but thank you!

  9. amistre64
    • 5 years ago
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    the only time your gonna have to actually worry about this stuff is in calculus classes when you try to manipulate formula into derivatives and integrals.... other than that....pointless. A calculator gives you all the answers

  10. anonymous
    • 5 years ago
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    ohh gotcha. but why do you use cos2x= cosxcosx - sinxsinx? i thought that was a sum and difference formula? im just a little confused with that part.

  11. amistre64
    • 5 years ago
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    since they want you to find cos(2x) and give you sin(x) its only natural :)

  12. anonymous
    • 5 years ago
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    oh, okay got it!

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