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hossam1122

  • 4 years ago

prove that sin54-sin18=1/2

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  1. GoldRush18
    • 4 years ago
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    0.80902-0.30902=0.5 or 1/2

  2. GoldRush18
    • 4 years ago
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    or should i say 0.5=1/2

  3. hossam1122
    • 4 years ago
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    @GoldRush18 u have to prove it using trigonometric functions and rules , not using calculator :D

  4. GoldRush18
    • 4 years ago
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    oh ok

  5. Ruka
    • 4 years ago
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    Use cofunction identity.

  6. hossam1122
    • 4 years ago
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    well i got cos36-sin18 and dont know what to do , can u prove it with explanation pls :)

  7. MelindaR
    • 4 years ago
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    Idk if this helps, but Sin(3theta)=3Sin(theta)-4(Sin(theta))^3 I tried using it and I got - 4 sin³ 18 + 2 sin 18 - 1/2 = 0

  8. blockcolder
    • 4 years ago
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    Use this identity: \[\sin A-\sin B=2 \cos{\frac{A+B}{2}}\sin{\frac{A-B}{2}}\]

  9. hossam1122
    • 4 years ago
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    thanks will do that :D @blockcolder

  10. hossam1122
    • 4 years ago
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    thanks i got it now it should be like this 2cos(36)sin(18) and multiply by cos18/cos18 as it equals 1 so cos(36)sin(36)/cos(18) and then multiply by 2 and 1/2 so 1/2sin72/cos18 then use co function so sin72= cos18 1/2cos(18)/cos(18) cosines cancel each other so = 1/2

  11. blockcolder
    • 4 years ago
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    Good job! :)

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