SOLUTION: solve the equation: 6cos^2(x)+8sin(x)=cos(2x)

Algebra ->  Trigonometry-basics -> SOLUTION: solve the equation: 6cos^2(x)+8sin(x)=cos(2x)      Log On


   



Question 1039806: solve the equation:
6cos^2(x)+8sin(x)=cos(2x)

Answer by ikleyn(52898) About Me  (Show Source):
You can put this solution on YOUR website!
.
solve the equation:
6cos^2(x)+8sin(x)=cos(2x)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~

As a first step, express all terms via sin(x):

6cos%5E2%28x%29 = 6%2A%281-sin%5E2%28x%29%29,

cos%282x%29 = cos%5E2%28x%29+-+sin%5E2%28x%29 = 1-sin%5E2%28x%29-sin%5E2%28x%29 = 1-2sin%5E2%28x%29.

Now substitute them into the original equation:

6%2A%281-sin%5E2%28x%29%29 + 8sin%28x%29 = {{1-2sin^2(x)}}}.

Simplify:

4sin%5E2%28x%29+-8sin%28x%29+-5 = 0.

Apply the quadratic formula:

sin%28x%29 = %288+%2B-+sqrt%2864%2B4%2A4%2A5%29%29%2F%282%2A4%29 = %288+%2B-+12%29%2F8.

Of the two values only sin(x) = -1%2F2 is acceptable.

Thus  the solutions are  x = %285pi%29%2F4  and/or  x = %287pi%29%2F4.