Right-click the slider above named 'a' and choose Animation On.
What features of the graph change and what features remain the
same?  The equation f(x) is displayed in the AlgebraView
window at left.  What is true about the equation when the graph
becomes a horizontal line for an instant?

a = -0.5 (slider from -4 to 4, step 0.5)
f(x) = (x - a) / (x - 1.0)