Problem I.1.6 mistake

[cspipaon]

In Problem I.1.6, in the second solution for D there is a mistake in the computation.

Let the corners of the parallelogram are: $A=(1,1)$,  $B=(1,3)$, $C=(4,2)$, the fourth corner is $D$. There are three possible values for $D$,
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* If $BD=AC$, then $D = B + BD = B + AC =  B + (C-A) = (4,2) + (3, 1) = (7, 3)$

In the last step B+(C-A) = (4,2) + (3, 1) = (7, 3), B=(1,3) seems to have been confused with C=(4,2).

I believe it should read B+(C-A) = (1,3) + (3, 1) = (4, 4)