Data Sufficiency

BTGmoderatorDC wrote: ↑

Thu Sep 16, 2021 11:51 am

cpotg_img4-300x285.png

In the diagram above, O is the center of the circle. What is the length of chord AC?

(1) chord BC = 14
(2) the circle has an area of 625π


OA C

Source: Magoosh

Target question: What is the length of chord AC?

Given: O is the center of the circle
If O is the center of the circle, then AB is the circle's DIAMETER
If AB is the DIAMETER, then ∠C = 90°, because ∠C is an inscribed angle containing ("holding") the diameter.
So, let's first add this information to the diagram


Statement 1: chord BC = 14
Notice that the length of chord BC has no bearing on the length of chord AC.
In fact, here are two diagrams that satisfy statement 1:


In the left-hand diagram, the answer to the target question is chord AC has length 20
In the right-hand diagram, the answer to the target question is chord AC has length 30
Since we cannot answer the target question with certainty, statement 1 is NOT SUFFICIENT

Statement 2: the circle has an area of 625π
Area of circle = πr²
So, we can write: πr² = 625π
Divide both sides by π to get: r² = 625
Solve: r = 25
So, the circle's radius = 25, which means the DIAMETER AB has length 50.

This time the length of the diameter has little bearing on the length of chord AC.
In fact, here are two diagrams that satisfy statement 2:


In the left-hand diagram, the answer to the target question is chord AC has length 30
In the right-hand diagram, the answer to the target question is chord AC has length 40
Since we cannot answer the target question with certainty, statement 2 is NOT SUFFICIENT

Statements 1 and 2 combined
When we combine the two statements, we see that we know the lengths of two sides of a RIGHT triangle


So, we COULD apply the Pythagorean Theorem to write: 14² + x² = 50²,
And we COULD solve the equation to get x = 48.
However, performing all of those calculations would be a waste of the time, since we need only show that we COULD answer the target question with certainty.
Since we COULD answer the target question with certainty, the combined statements are SUFFICIENT

Answer: C

Cheers,
Brent