Question 146817
Opposite corners are (-2,5) and (4,-1),
and (-2,-1) and (4,5). 
Find the slope m of the lines connecting opposite corners.
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(-2,5) and (4,-1)
m1 = (y2-y1)/(x2-x1)
m1 = (-1-5)/(4+2)
m1 = -6/6 = -1
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(-2,-1) and (4,5)
m2 = (5 - (-1))/(4 -(-2))
m2 = 6/6 = +1
Since the slopes are negative inverses, they're perpendicular.