In the figure, YX−→− and YZ−→− are tangents to circle A at points X and Z, respectively.
mXZ =140∘ mXSZ =220∘
What is the measure of ∠XYZ ?
We have this
m XYZ = (1/2) (m XSZ - m XZ ) = (1/2) ( 220 - 140 ) = (1/2) (80) = 40°
+121 
