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Homework Help: Algebraic solution of Trig equation.

  1. Mar 22, 2008 #1
    The question as is written :

    Solve for x

    5cosx-2= -0.72 ( 0 [tex]\leq[/tex] x [tex]\leq[/tex] 2[tex]\Pi[/tex] )

    (The answer given is "1.31, 4.97".)

    So, i done what seemed natural.


    5cosx = 1.28
    cosx = 1.28/5
    cosx = 0.256

    Firstly, i'm not even sure what unit my answer is supposed to be in. Since x is defined as Radians in the question, my answer of .256 = .256pi Radians (.256 of 180)?
    Which should give solutions in the 1st & 4th quadrants.

    All of the questions untill (there are a bunch similar to this) these ones have been in the form of, for example, 4=5 - tan2x (x in radians), which, as there are no decimals in the solution work out easy (pi/8, 5pi/8).. . why, if x is defined in Radians in the question, are the answers in decimals? I dont understand the principle of this when decimals are involved for some reasons.
  2. jcsd
  3. Mar 22, 2008 #2

    Should be in radians.

    Find the values in the 1st and 4th quadrant.
  4. Mar 22, 2008 #3


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    Homework Helper

    Well if 5-tan2x=4 then that is the same as tan2x=1,right? and arctan(1)=pi/4

    The only reason why are usually get your answers in terms of pi are most likely due to the fact that the questions you do end up with a trig function being equal to a standard number.

    For example, [itex]sin3x=\frac{\sqrt{3}}{2} \Rightarrow 3x=sin^{-1}(\frac{\sqrt{3}}{2})[/itex]

    and [itex]sin^{-1}(\frac{\sqrt{3}}{2})=\frac{\pi}{3}[/itex]
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