If f is differentiable at c and f′(c) = 0, then we call c a critical point or
stationary point of f. A point c at which the derivative of f is not defined is called a
singular point of f.
Thus we know that the candidates for the location of the extreme values of a continuous
function on a closed interval fall into three categories: (a) endpoints of the interval, (b)
critical points, and (c) singular points. To determine the extreme values of such a function
f, we identify all these points, evaluate f <span>at each one, and identify the largest and smallest
values. </span>
The chords ST and RA intesect at Y, so that SY is now perpendicular to RY and they form an angle 90 degrees at that point. However angles mSA and mRT are both at the circumference of the circle(a chord is a line from point of a circle's circumference to the other) and are both 90 degrees because angle at the circumference is half of angle at the centre in same arc.
