###### Asked in Math and Arithmetic, Algebra, Calculus

# How do you solve the equation for x 2cos x squared plus 3sin x equals 0?

## Answer

###### Wiki User

###### 04/28/2010

Well, let's see.

**2 cos****2****(x) + 3 sin(x) = 0**

We're going to use the only powerful magic trick we know right now, right at the beginning.

Then, after that, we'll have no tricks left, and it'll just be a slog from there to the end.

The Trick: [ **cos****2****(x)** ] is the same thing as
[ **1 - sin****2****(x)** ]. That's the only trig trick we
have.

Substitute that in the original equation where you see cos2(x) :

**2 [ 1 - sin****2****(x) ] + 3 sin(x) = 0**

Now, since we have a lot of slogging to do, let's make it a little easier on ourselves.

Wherever we have sin(x), let's write 'S' instead. At the end, when we figure out what 'S' is,

we'll go back to calling it "sin(x)" again .

**2 (1 - S****2****) + 3 S = 0** . . . (See ? Isn't
that a lot easier to write ?)

Eliminate the parentheses on the left side:

2 - 2 S2 + 3 S = 0

Re-arrange it a little:

-2 S2 + 3 S + 2 = 0

Just for convenience, multiply each side by -1:

2 S2 - 3 S - 2 = 0

Factor the left side:

(2S + 1) (S - 2) = 0

If either factor is zero, we get a solution for the equation:

2S + 1 = 0 . . . . . 2S = -1 . . . . . **S =
-1/2**

and

S - 2 = 0 . . . . . **S = 2**

Now that we have actual numbers for 'S', we can call it "sin(x)" again,

and figure out what the angle 'x' is.

The first solution: sin(x) = -1/2 . . . . . **x = 210° ,
330°**

Second solution: sin(x) = 2 . . . . . That's a nonsense solution. The sine of anything can't be greater than 1.

So we're done.