This is not true.  This is because when squaring both sides and then plugging back into the original equations, some of our solutions may be extraneous solutions.  Therefore, when solving a trigonometric equation by squaring both sides, all solutions found must be plugged back into the original equation and validated.

We should only square by both sides when all other identities are not able to be used in an equation.  Quite often, you will find that a trigonometric identity can be used to simplify an equation.  Squaring both sides is ultimately trying to produce a trigonometric identity in order to solve for the equation.

We begin with our original equation:

                 (Pythagorean Identity)

Looking at the unit circle we see that  at  and .  We must plug these back into our original equation to validate them.

Checking 

Checking 

And so our only solution is 

We begin with our original equation:

               (Pythagorean Identity)

From the unit circle, we see that . We must check both of these solutions in the original equation.

Checking 

Checking  

So we see our only solution is 

We begin with our original equation

                                  (Pythagorean Identity)

                                                                                            (substitution)

Using this form, we see we really only need to consider when   at   and .  Now we must plug these values into the original equation to check and see if they are both acceptable solutions to our problem.

Checking :

Checking 

By checking our solutions we see the only solution to our equation is .

We begin with our original equation.

                                             (Pythagorean Identity)

                                                                    (Double-Angle Formula)

We know that  will be equal to  for when  is any multiple of and when .  We need to check both solutions (we will simply check  for simplicity) to make sure they are valid solutions.

Checking :

Checking 

By checking our solutions, we see the only solution to this equation is 

This one is not as straight-forward.  We must manipulate the original equation before squaring both sides.

                                     (Pythagorean Identity)

                                                                 (divide both sides by 2)

Solving for each:

   radians

Or 

From the unit circle we know that  when .

So now we must go back and check all of our solutions.

Checking 

Checking  (this is also equal to checking )

Both of our solutions are correct.

Our first line of defense when solving trigonometric functions is using a familiar identity/formula such as the Pythagorean Identities or the Double Angle Formulas.  When we are unable to use an identity or formula we are able to square both sides of the equation and with further manipulation we are usually able to produce one of these identities thus simplifying our problem and making it easier to solve.