Simplify
sin(180-theater).cos(90°-theater)-1 /cos(-theater)

[sin(180-a) x cos(90-a) - 1]/cos(-a)
= [sin(a) x sin(a) - 1]/(-cos(a))
= [sin^2(a) -1]/(-cos(a))
= [-cos^2(a)]/(-cos(a))
= cos(a)

formula used
sin(180-a) = sin(a)
cos(90-a) = sin(a)
cos(-a) = -cos(a)
sin^2(a) + cos^2(a) = 1
all of these are easily proved by the unit circle. you might find someone to explain this. it will be very handy.

Bis your answer. it only holds the decimal numerals