Results 1 to 8 of 8

Thread: Trig

  1. #1
    Junior Member R3ap3r's Avatar
    Joined
    Apr 2008
    Posts
    58

    Trig

    If $\displaystyle cos\beta = -\frac{3}{11}$ and $\displaystyle \beta$ is in the second quadrant, find the exact value of $\displaystyle sin\;2\beta$

    Well $\displaystyle sin2\beta = 2sin\beta cos\beta$

    My final answer is $\displaystyle -5.77$ is this correct?
    Follow Math Help Forum on Facebook and Google+

  2. #2
    Moo
    Moo is offline
    A Cute Angle Moo's Avatar
    Joined
    Mar 2008
    From
    P(I'm here)=1/3, P(I'm there)=t+1/3
    Posts
    5,618
    Thanks
    6
    Quote Originally Posted by R3ap3r View Post
    If $\displaystyle cos\beta = -\frac{3}{11}$ and $\displaystyle \beta$ is in the second quadrant, find the exact value of $\displaystyle sin\;2\beta$

    Well $\displaystyle sin2\beta = 2sin\beta cos\beta$

    My final answer is $\displaystyle -5.77$ is this correct?
    Hullo,

    A sine can barely be <-1 :/

    Here is how to do. You know that $\displaystyle \beta$ is in the second quadrant, so $\displaystyle \sin(\theta)>0$ and $\displaystyle \cos(\theta)<0$

    We know that $\displaystyle \cos^2(\beta)+\sin^2(\beta)=1$

    --> $\displaystyle \sin^2(\beta)=1-\left(-\frac{3}{11}\right)^2=\frac{112}{121}$

    Since $\displaystyle \sin(\beta)>0$, $\displaystyle \sin(\beta)=\sqrt{\frac{112}{121}} \approx \dots$


    Hence, $\displaystyle \sin(2 \beta)=\dots$
    Follow Math Help Forum on Facebook and Google+

  3. #3
    Junior Member R3ap3r's Avatar
    Joined
    Apr 2008
    Posts
    58
    Well I know atleast what i did wrong. i forgot to put $\displaystyle \sqrt{112}$ over 11 in the formula.

    New answer is $\displaystyle -.525$ hows this look?
    Follow Math Help Forum on Facebook and Google+

  4. #4
    Junior Member R3ap3r's Avatar
    Joined
    Apr 2008
    Posts
    58
    Quote Originally Posted by janvdl View Post
    Code:
       |\
       | \
       |  \  sqrt{130}
    11 |   \
       |__o_\
       
        3
    That $\displaystyle o$ there represents Beta.

    We wish to find $\displaystyle 2 Sin \beta Cos \beta$

    = $\displaystyle 2 \left( \frac{11}{ \sqrt{130} } \right) \left( \frac{3}{ \sqrt{130} } \right) $

    = $\displaystyle \frac{33}{65}$
    I believe you made an error. For one its the $\displaystyle \sqrt{112}$ and my math is $\displaystyle 2\cdot \left(\frac{\sqrt{112}}{11}\right)\left(\frac{-3}{11}\right)$

    whose right? lol
    Follow Math Help Forum on Facebook and Google+

  5. #5
    Moo
    Moo is offline
    A Cute Angle Moo's Avatar
    Joined
    Mar 2008
    From
    P(I'm here)=1/3, P(I'm there)=t+1/3
    Posts
    5,618
    Thanks
    6
    Quote Originally Posted by R3ap3r View Post
    I believe you made an error. For one its the $\displaystyle \sqrt{112}$ and my math is $\displaystyle 2\cdot \left(\frac{\sqrt{112}}{11}\right)\left(\frac{-3}{11}\right)$

    whose right? lol
    I'm pretty sure it's this one
    Follow Math Help Forum on Facebook and Google+

  6. #6
    o_O
    o_O is offline
    Primero Espada
    o_O's Avatar
    Joined
    Mar 2008
    From
    Canada
    Posts
    1,410
    Thanks
    1
    In Janvdl's diagram, the 11 should be the hypotenuse.
    Follow Math Help Forum on Facebook and Google+

  7. #7
    Bar0n janvdl's Avatar
    Joined
    Apr 2007
    From
    Meh
    Posts
    1,630
    Thanks
    6
    Quote Originally Posted by R3ap3r View Post
    I believe you made an error. For one its the $\displaystyle \sqrt{112}$ and my math is $\displaystyle 2\cdot \left(\frac{\sqrt{112}}{11}\right)\left(\frac{-3}{11}\right)$

    whose right? lol
    Moo's. And I'm going to bed because I can't seem to concentrate anymore. Sorry for the mistake Reap3r.
    Follow Math Help Forum on Facebook and Google+

  8. #8
    Junior Member R3ap3r's Avatar
    Joined
    Apr 2008
    Posts
    58
    np
    Follow Math Help Forum on Facebook and Google+

Similar Math Help Forum Discussions

  1. Compute Trig Function Values, Solve Trig Equation
    Posted in the Trigonometry Forum
    Replies: 1
    Last Post: Sep 8th 2011, 07:00 PM
  2. Replies: 7
    Last Post: Apr 15th 2010, 08:12 PM
  3. Replies: 6
    Last Post: Nov 20th 2009, 04:27 PM
  4. Replies: 1
    Last Post: Jul 24th 2009, 02:29 AM
  5. Trig Equations with Multiple Trig Functions cont.
    Posted in the Trigonometry Forum
    Replies: 4
    Last Post: Apr 7th 2008, 05:50 PM

Search Tags


/mathhelpforum @mathhelpforum