The solution to this problem on chegg and in the book is 4 a

The solution to this problem, on chegg and in the book, is ?/4 and 5?/4. Can someone explain why 3?/4 and 7?/4 isnt part of the answer. Thanks!

Solution

cos =-sin(-)

=>cos =-(-sin()) [since sin(-x) =-sin(x)]

=>cos = sin()

=>=/4 and 5/4

for = 3/4 and 7/4 cos =- sin() but not cos =(-sin(-))