MA represents a vertical tower. Points A, B, and C lie in the same horizontal plane. MB and MC are wire stays, each of length 20 meters. The angle of elevation of M at B is *x*°. ∠MCB is *z*°, and ∠ABC = *y°* = ∠ACB.

- Prove that BC = 40 cos
*z*. - Show that BC = 40(cos
*x*)(cos*y*). - Calculate the magnitude of
*x*if*y*= 42� and*z*= 71.8�.