To critique Opal's reasoning, let's focus on the claim being made: "Because corresponding sides of similar triangles are proportional, ________________." In the context of similar triangles, the statement must express a relationship that leads to the conclusion about the sides.
The most appropriate option to fill in the blank would be:
"DF/BC = 1/2."
Here’s a breakdown of the reasoning:
- Since triangles \( \triangle ADF \sim \triangle ABC \), it follows that corresponding sides are proportional. This means that: \[ \frac{DF}{BC} = k \text{ (for some constant } k\text{)} \]
- If they suspect \( DF \) is exactly half the length of \( BC \), then \( k \) would be \( \frac{1}{2} \), which gives: \[ \frac{DF}{BC} = \frac{1}{2} \]
- Thus, multiplying both sides by \( BC \) leads to: \[ DF = \frac{1}{2} BC \]
So in conclusion, the correct option to fill in their blank is: "DF/BC = 1/2."
This accurately sets up the proportional relationship needed in their proof. The other options either do not express the required relationship or are phrased incorrectly for the context of similar triangles.