Sir in example 12 before 6.2 , AT X=π/6 f'(x) =0 , so we take big brac

This question was asked in Ex 6.2,4

Sir in example 12 before 6.2 , AT X=π/6 f'(x) =0 , so we take big bracket ....but in question 4 of 6.2 at x=3/4 f'(x) =0 but we not include big bracket.....why??????