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Math Help - Inverse trigonometric functions

  1. #1
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    Inverse trigonometric functions

    Hello ,

    solve cosx = 2/7 for x [Pie , 2Pie]

    My solution is first : Arc cos ==> 1.28 rd , 1.28/3.14 = 0.4076 Pie its not in the domain so what to do ? I don't know what to do after that ? please can you help me ?

    Thanks
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  2. #2
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    Hello, iceman1!

    By the way, it's not "pie" . . . it's "pi".


    Solve: . \cos x \:=\:\tfrac{2}{7}\quad\text{for }[\pi,\:2\pi]

    My solution is first: . \arccos\left(\tfrac{2}{7}\right) \:=\:1.28\text{ rd} \quad\Rightarrow\quad \frac{1.28}{\pi} . ?? . Why?
    An inverse trig function has an infinite number of values.

    . . \arccos\left(\tfrac{2}{7}\right) \;=\;\pm1.281 + 2\pi n\;\;\text{ for any integer }n


    Now find n so that the angle falls within the interval.

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  3. #3
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    oh thanks yeah sorry ''PI'' . 1.28/pi to get the result is PI ? coz its in Radian .

    anway how to find n ? I need an exemple how to resolve this problem .

    our teacher is drawing the circle and i m lost sometimes she adjust -Pi -x , +Pi + x Please can you help me how to know ?

    Thanks
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  4. #4
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    Quote Originally Posted by iceman1 View Post
    oh thanks yeah sorry ''PI'' . 1.28/pi to get the result is PI ? coz its in Radian .

    anway how to find n ? I need an exemple how to resolve this problem .

    our teacher is drawing the circle and i m lost sometimes she adjust -Pi -x , +Pi + x Please can you help me how to know ?

    Thanks
    Soroban told you that " <br />
\arccos\left(\tfrac{2}{7}\right) \;=\;\pm1.281 + 2\pi n\;\;\text{ for any integer }n<br />
    When n is 0, that is 1.281 radians. When n= 1, that is 1.281+ 2(3.141)= 7.563 and 1.281- 2(3.141)= -5.002. When n= 2, ... Do the arithmetic!
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