Grogg drew a circle. Then, he drew a straight line inside the circle and filled in the region above it, like so: [asy]
size(200);
draw(unitcircle);

pair A = dir(90 - 50);
pair B = dir(90 + 50);

draw(A--B, red);

currentpicture = shift(-3, 0) * currentpicture;

draw(unitcircle);
path cap = arc(origin, A, B);
cap = cap--cycle;
filldraw(cap, purple);
draw(A--B, red);

[/asy]

What information would we need to know in order to calculate the area of the filled in purple region?

Can you draw an example where you can calculate the area?