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Show that function f(x)=sin(2x+pi/4) is ...

Show that function `f(x)=sin(2x+pi/4)` is decreasing on `((3pi)/8,(5pi)/8)dot`

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Here, `f(x) = sin(2x+pi/4)`
`:. f'(x) = cos(2x+pi/4)*2`
`=>f'(x) = 2cos(2x+pi/4)`
Here, `(3pi)/8 lt x lt (5pi)/8`
`=> pi lt (2x+pi/4) lt (3pi)/2`
Now, `cos(2x+pi/4) ` will be negative in this interval.
`:. f'(x) lt 0`.
It means `f(x)` is a decreasing function on `((3pi)/8,(5pi)/8)`.
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