Question 965883
Let the rectangle have length a and width b. 
One vertex is at ({{{0}}},{{{0}}}).
One at ({{{a}}},{{{0}}}).
One at ({{{0}}},{{{b}}}).
One at ({{{a}}},{{{b}}}).
The two points that join one diagonal are ({{{0}}},{{{0}}}) and ({{{a}}},{{{b}}}.
The slope is,
{{{m[1]=(b-0)/(a-0)=b/a}}}
The two points that join the other diagonal are ({{{0}}},{{{b}}}) and ({{{a}}},{{{0}}}).
That slope is,
{{{m[2]=(0-b)/(a-0)=-b/a}}}
.
.
So,
{{{m[2]=-m[1]}}}