sec(780°) + cos(-540°) + sin(630°)

Any angle which is more than 360° can be reduced to a
smaller angle with the same trig functions by dividing 
by 360° and taking the remainder, so 

      2               1               1 
360)780         360)540         360)630
    720             360             360
     60             180             270

So the problem reduces to

sec(60°) + cos(-180°) + sin(270°) 

Since cos(-q) = cos(q),

sec(60°) + cos(180°) + sin(270°)

The secant is the reciprocal of the cosine and cos(60°) = 1/2. So
sec(60°) = 2, cos(180°) = -1 and sin(270°) = -1, and the above is

2 + (-1) + (-1) = 0

Edwin