MATLAB Code:
    Plot a unit circle centered at the origin with line
    Plot a unit circle centered at the origin with plot
    Plot a unit circle with specified center
    Plot two circles with specified radii, tangent at origin, centers on x axis
    Alternative to above, two calls to plot, colors specified for circles
    Plot two circles, specified radii, tangent at origin, centers on x axis, and third circle, specified radius, tangent to others