If A, B and C are interior angles of a triangle ABC, then 
=.
Given,
A, B and C are the interior angles of ∆ ABC,
Therefore,
A + B + C = 180°
B + C = 180° - A
Now put this value in the given equation we get,
[∵ sin (90 – θ) = cos θ]
30
If A, B and C are interior angles of a triangle ABC, then =.
Given,
A, B and C are the interior angles of ∆ ABC,
Therefore,
A + B + C = 180°
B + C = 180° - A
Now put this value in the given equation we get,
[∵ sin (90 – θ) = cos θ]