please someone help me...

Step-by-step explanation:
x cos θ + y sin θ = x cos β + y sin β
Let's say x = cos A, and y = sin A.
cos A cos θ + sin A sin θ = cos A cos β + sin A sin β
Use angle sum formula.
cos(A − θ) = cos(A − β)
A − θ = ±(A − β)
A − θ = β − A
2A = θ + β
A = (θ + β) / 2
cot A = cot((θ + β) / 2)
x/y = cot((θ + β) / 2)
Take a look at the explanation below. It proves that 'x / y = cot(θ + β / 2)' using a different method.