Question

are the lines y=x and y+x=0 perpendicular

Answer 1

yes they are perpendicular as they bisect each other

Answer 2

yes because the product of the slopes should be -1 if the two lines are perpendicular.
y=x
y-x=0
m1=-1
y+x=0
m2=1
the product of m1 and m2 is -1 so the two lines are perpinducular
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