Question 213979
Find the coodinates of the missing endpoint given that S is the midpoint of RT. 
T(-4,3),S(-1,5)
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Let point "R" have coordinates (x,y)
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(x +(-4))/2 = -1
x-4 = -2
x = 2
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(y+3)/2 = 5
y+3 = 10
y = 7
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Ans: R is (2,7)
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Note: The coordinates of the midpoint are the average of the 
corresponding coordinates of the two endpoints.
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Cheers,
Stan H.