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Homework Help: Trigonometry question

  1. Jan 17, 2007 #1
    The voltage E applied to the sending end of a high-pressure transmitting line is connected to the voltage e at the receiving end by the equation
    E^2=(e*cos(x) + a)^2 + (e*sin(x) + b)^2, where a and b are constants. Expand the right-hand side of the equation and by expressing a*cos(x) + b*sin(x) in the form R*cos(x + alpha) show that the maximum and minimum values of R, as x varies, are e +/-sqr(a^2 + b^2)? On expanding I get the following:

    E^2 = e^2*((cos(x))^2 + (sin(x))^2) + a^2 + b^2 + 2*e*(a*cos(x) + b*sin(x))
    E^2 = e^2 + a^2 + b^2 + 2*e*sqr(a^2 + b^2)*(cos(alpha)*cos(x) + sin(alpha)*sin(x)); where,
    cos(alpha) = a/sqr(a^2 + b^2), and sin(alpha) = b/sqr(a^2 + b^2), and tan(alpha) = b/a. Therefore
    E^2 = e^2 + a^2 + b^2 +2*e*sqr(a^2 + b^2)(cos(x - alpha)), so I get R = 2*e*sqr(a^2 + b^2) not what it is claimed above. What is the next step? Many thanks.
  2. jcsd
  3. Jan 18, 2007 #2
    Let me tidy up your equation.. otherwise no one is able to read that.....

    Okay, I've finished my part... someone gonna help him.....
    Last edited: Jan 18, 2007
  4. Jan 18, 2007 #3
    I believe you mis-understand what the question is asking......

    The question is asking you to express [tex]acos(x) + bsin(x)[/tex] in term of [tex] Rcos(x+\alpha)[/tex], then show the maximum and minimum value of E is [tex] e+/-\sqrt{a^2+b^2}[/tex], That would be more make sense....
    Last edited: Jan 18, 2007
  5. Jan 19, 2007 #4
    Thanks for taking the time to make my post readable chanvincent.
  6. Jan 19, 2007 #5
    Expanding [tex] a cos(x) + b sin(x) [/]tex] I get :
    [tex] = \sqrt{a^2 + b^2}(a/\sqrt{a^2 + b^2}cos(x) + b/\sqrt{a^2 + b^2}sin(x))
    = \sqrt{a^2 + b^2}(sin(\alpha)cos(x) + cos(\alpha)sin(x))[/tex], where [tex] \(alpha)=tan(a/b)[/tex], this expands out to: [tex] \sqrt{a^2 + b ^2}(1/2( sin(\(alpha) + x) + sin(\alpha - x)) + 1/2( sin(\alpha + x) - sin(\alpha -x)))[/tex]
    , which reduces to [tex] \sqrt{a^2 + b^2}(sin(\alpha + x))[/tex].The question is how do you expand it to get [tex] R cos(x + \alpha)[\tex], and what do you do then.

    (Edited by HallsofIvy to fix tex. John, you put [ \tex ] when you should have [ /tex ] to end the tex.)
    Last edited by a moderator: Jan 19, 2007
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