OE
[STEP 1]: Analyze the centered information and the quantities.
In this problem, you are given additional information: the sentence that tells you the diameter of circle O is d and the area is a. This is important information because it gives you a key to unlocking this question. Given that information, you can tell that you are comparing the area, a, of circle O and a quantity that includes the diameter of the same circle. If you’re thinking about the formula for calculating area given the diameter, you’re thinking right!
[STEP 2]: Approach strategically.
Make Quantity B look more like Quantity A by rewriting a, the area of the circle, in terms of the diameter, d. The area of any circle equals \(πr^2\), where r is the radius. Because the radius is half the diameter, you can substitute d/2 for r in the area formula to get \(a= πr^2 = π (\frac{d}{2})^2\) in Quantity B. Simplifying, you get \(\frac{πd^2}{4.}\)
Because both quantities contain π, we could compare \(\frac{d^2}{2}\) to \(\frac{d^2}{4}\) . But let’s take it one step further. You know that d is a distance and must be a positive number. That makes it possible to divide both quantities \(\frac{d^2}{2}\) and \(\frac{d^2}{4}\), and by \(d^2 \)and then just compare 1/2 to 1/4. This makes it easy to see that Quantity A is always greater because \(\frac{1}{2}>\frac{1}{4}.\)
Choice (A) is correct.