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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