In the solution of the following question , I want to know how the marked (in blue bracket) step came - Maths - Application of Derivatives

Question

In the solution of the following question , I want to know
how the marked (in blue bracket) step came ?

Q Find the intervals in which f x   =   sin 3 x   -
  cos 3 x   ,   0 <   x   <  
π is strictly increasing or strictly decreasing?

Neha Sethi · Accepted Answer

Dear student We have :sin3x+π4>0⇒2π<3x+π4<3πThis is so because graph of sine function is positive between intervals 0,π ,2π,3π, and sine function is negative between intervalsπ,2π,3π,4π, So for sin3x+π4>0you can also consider⇒0<3x+π4<π Regards

In the solution of the following question , I want to know how the marked (in blue bracket) step came ? Q. Find the intervals in which f x = sin 3 x - cos 3 x , 0 < x < π is strictly increasing or strictly decreasing?

In the solution of the following question , I want to know how the marked (in blue bracket) step came ?

Q. Find the intervals in which $f (x) = \sin 3 x - \cos 3 x, 0 < x < π$ is strictly increasing or strictly decreasing?