Circle X is shown in the diagram.

Circle X is shown. 2 chords intersect at a point to form arcs a and b. Arc a is intercepted by angle 1. Arc b is intercepted by angle 2. Arcs d and c are the other 2 arcs.

Which equation can be used to solve for m∠1?

m∠1 = One-half(a – b)
m∠1 = One-half(a + b)
m∠1 = One-half(c – d)
m∠1 = One-half(c + d)