Step-by-step explanation:
The slope of PQ is:
m = [(c + a) − (b + c)] / (b − a)
m = (c + a − b − c) / (b − a)
m = (a − b) / (b − a)
m = -1
The slope of QR is:
m = [(a + b) − (c + a)] / (c − b)
m = (a + b − c − a) / (c − b)
m = (b − c) / (c − b)
m = -1
Both lines have the same slope, and therefore are parallel.
