Answer:
D(4 ; 1)
Explanation:
vector(AB)=(2-1 ; 3-1) = (1 ; 2)
vector(DC)=(5-x ; 3-y) and D(x ; y )
ABCD parallelogram :vector(AB)=vector(DC)
you have the system : 5-x =1
3-y =2
so : x=4 and y=1
D(4, 1)
The two diagonals have the same midpoint, so ...
(A+C)/2 = (B+D)/2
A+C = B+D . . . . multiply by 2
A+C-B = D . . . . . subtract B
D = (1, 1) + (5, 3) - (2, 3) = (1+5-2, 1+3-3)
D = (4, 1)
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