SOLUTION: Please solve the equation for the interval [ 0, 2pi ]: {{{2cos^2(2x)-cos(2x)=0}}} Thank you!

Algebra ->  Trigonometry-basics -> SOLUTION: Please solve the equation for the interval [ 0, 2pi ]: {{{2cos^2(2x)-cos(2x)=0}}} Thank you!      Log On


   



Question 983734: Please solve the equation for the interval [ 0, 2pi ]:
2cos%5E2%282x%29-cos%282x%29=0
Thank you!

Answer by ikleyn(52790) About Me  (Show Source):
You can put this solution on YOUR website!

One solution is

cos%282x%29 = 0.

It gives   2x = pi%2F2+%2B+k%2Api,  where  k  is an arbitrary integer  (k = 0, +/-1, +/-2, . . . ).

Hence  x = pi%2F4+%2B+k%2A%28pi%2F2%29.

It gives for  k = 0, 1, 2, 3

x = pi%2F4,  pi%2F4+%2B+pi%2F2 = 3pi%2F4,  pi%2F4+%2B+pi = 5pi%2F4,  pi%2F4+%2B+3pi%2F2 = 7pi%2F4.

Next,  if  cos%282x%29 is not zero,  then you can reduce the original equation to the form

cos%282x%29 = 1%2F2.

dividing it by 2cos%282x%29.

It gives you

2x = +/-pi%2F3 + 2k%2Api,  where  k  is an arbitrary integer  (k = 0, +/-1, +/-2, . . . ).

Hence  x = +/-pi%2F6 + k%2Api%29.

It gives for  k = 0, 1, 2, 3,

x = pi%2F6,  pi%2F6+%2B+pi = 7pi%2F6,  -pi%2F6+,  -pi%2F6+%2B+pi = 5pi%2F6.

Answer. x = pi%2F4,  3pi%2F4,  5pi%2F4,  7pi%2F4.

             x = pi%2F6,  7pi%2F6,  -pi%2F6,  5pi%2F6.