Function or not a function.
2 answers:
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If lines through XY and YW form a 90° angle, so that Y is a right angle,
then the product of the slopes of the lines through XY and YW must be -1
let the slopes through X,Y and Y,W be
and 
respectively
so
Right answer:
then the product of the slopes of the lines through XY and YW must be -1
let the slopes through X,Y and Y,W be
so
Right answer: