3. (a) Write down the equation of the straight line through the point $( 1,2 )$ with slope - 1 .
(b) Let $l$ be a line with equation
$$y = ( 2 - a ) + a x$$
where $a$ is a constant. Show that, for any $a$, the line passes through the point $( 1,2 )$. Find the equation of the line perpendicular to this line which also passes through the point $( 1,2 )$.
(c) Find the equations of the lines which pass through the point $( 1,2 )$ and have perpendicular distance 1 from the origin.