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Homework Help: Maths - trig

Posted by Rose on Sunday, October 3, 2010 at 5:37pm.

Solve 3cos2x - 7cosx = 0, when 0<=x<=360

And also, find the exact values for x when 0<=x<=360 if 3tan^2x=1


Thankyou!

• Maths - trig - Reiny, Sunday, October 3, 2010 at 7:05pm

  have you come across
  cos 2x = 2cos^2 x - 1 ?
  let's sub it.
  3cos 2x - 7cosx = 0
  3(2cos^2 x - 1) - 7cosx = 0
  6cos^2x - 7cosx - 3 = 0
  (2cosx-3)(3cosx+1) = 0
  
  cosx = 3/2 , which is not possible,
  or
  cosx = -1/3
  so x must be in quadrants II or III
  angle in standard position is 7.53 , then
  x = 180-70.53 = 109.47°
  or
  x = 180+70.53 = 250.53
  

• 2nd part Maths - trig - Reiny, Sunday, October 3, 2010 at 7:09pm

  3tan^2 = 1
  tan^2x = 1/3
  tanx = ± 1/√3
  
  x = 30°, 150°, 210° or 330°
  

• ìàòåìàòèêà - Anonymous, Tuesday, March 1, 2011 at 3:47am

  3cos2x+7cosx+3=0
  

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