So m>0, the solution is: m= 1/2.

2.cosa= root sign (1-sin 2), which obviously cannot be guaranteed.

Tana=sina/cosa, obviously not.

Sin2a=2sinacosa, also related to cosa, No.

cos2a=( 1-2sina^2),ok！

3.sin(x-5π/2)=sin(x-π/2)=-cosx，

So f(x)=-x*cosx.

f(-x)=x*cosx=-f(x)

So f(x) is odd function.