For a concave mirror, if real image is formed the graph between 1u and 1vis of the form

since 1f=+1v+1u⇒1v=-1u+1f Putting the sign convention properly 1-v=-1-u+1-f⇒1v=-1u+1f comparing this equation with y=mx+c slope = m= tanθ=-1⇒135o or -45o and intercept C=+1f