First correct response in the comments gets a free Math Guide. Usual contest rules apply. Ready? Here we go!
MUAHAHAHA. Good luck!
UPDATE: Tuấn got it first, although a others came VERY close (careful with parentheses!!!). Solution below the cut.
It might not be immediately clear what to do here, but as is true of most geometry questions, it's a good idea to start by listing everything you know. First, you know that triangle ABC is a right triangle, because BC is a diameter, and point A is on the circle. You might recall from geometry class that if an angle lies on a circle, its measure is half of the measure of the arc it corresponds to. Since A is on the circle and corresponds to a 180º arc, the measure of angle A is 90º. You probably won't see that rule tested on the SAT, but it's not outside the realm of possibility. So here's what we know:
Of course, we also know that the formula for the area of a triangle is ½bh. So let's figure out AB:
Oh look. Difference of two squares. Crazy how often that appears!
Now we can solve for BC, using the Pythagorean Theorem:
Nice. Here's what we've got now:
Of course, BC is the diameter of the circle, and to find the circle's area we want the radius, which is half of BC:
Now, about that area: