tangent perpendicular to radius.
tangents meet at a point equidistant from circumference. (isos. Triangle)
alt seg thm
angle at ctr = 2 angle at circumf. . .
y-axis is x=0. So, lines parallel to y axis are x =? . . .
x-axis is y=0.
parallel lines have same gradient.
product of gradient of perpendicular lines is -1.