Therefore
f ′(x) > 0  if cos x – sin x > 0
or
f ′(x) > 0  if cos x > sin x  or cot x > 1
Now
cot x > 1 if tan x < 1, i e , if 0
4
x
π
<
<
Thus
f ′(x) > 0  in 0 4
, π


Hence f is increasing function in 0, 4
π





 Example 35 A circular disc of radius 3 cm is being heated