Explanation:
supplement means their addition is equal to 180°
since m<x and m<y are supplement then m<x+m<y=180
and <y supplement to <z (their addition equals to 180°)
since m<y and m<z are supplement then m<y+m<z=180
m<x+m<y= m<y+m<z
m<y cancelled
m<x= m<z
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FOR EXAMPLE
Given that <x and <y are supplement
and <y and <z are supplement
if <x= 45°
that means that <y = 135°
so that <x +<y = 180°
45+135= 180 ✓
and <y and <z are supplement
m<y+ m<z = 180°
then m<z= 45°
m<x+m<y= m<y+m<z
m<x +135= 135+m<z
then m<x= m<z = 45°