Answer:
(a + b)/2 = x
(b + c)/2 = y
(a + c)/2 = z
Adding these three equations:
(a + b + b + c + a + c)/2 = x + y + z
(2a + 2b + 2c)/2 = x + y + z
a + b + c = x + y + z
(x + y + z)/3 = (a + b + c)/3
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