Let's address the two math questions in your image:

Câu 17:

The problem involves two similar triangles $A'B'C'$ and $ABC$, with the goal to find the correct ratio of corresponding sides that maintain the similarity condition. The answer choices involve the ratio of different sides of these triangles:

A. $\frac{AB}{A'B'} = 3$

B. $\frac{AB}{A'C'} = 3$

C. $\frac{A'B'}{AB} = 3$

D. $\frac{A'B'}{AC} = 3$

To determine the correct ratio, recall that the corresponding sides of similar triangles are proportional. The correct ratio will be between the corresponding sides of $ABC$ and $A'B'C'$. Based on the typical notation, the ratio $\frac{AB}{A'B'} = 3$ seems likely (A), as it follows the conventional order for comparing corresponding sides.

Câu 18:

This question involves two points $M$ and $N$ lying on the sides $BC$ and $AC$, respectively, of triangle $ABC$. We are asked to select the correct statement based on the information that $MN \parallel AB$, which implies certain proportional relationships and geometric properties. The answer choices are:

A. $\triangle AMN$ is similar to $\triangle ABC$

B. $\triangle ABC$ is similar to $\triangle MN$

C. $\triangle NMC$ is similar to $\triangle ABC$

D. $\triangle CAB$ is similar to $\triangle CMN$

When two lines are parallel and cut by transversals, the resulting triangles are similar. Here, since $MN \parallel AB$, the triangles formed by the segments $MN$ and $AB$ will be similar. The correct answer is likely A, since $\triangle AMN$ and $\triangle ABC$ will share corresponding angles.

Would you like more detailed explanations or help with other related problems? Here's a quick list of related questions for further study:

1. How do you prove the similarity of two triangles?
2. What is the property of parallel lines in triangle similarity?
3. How can you use ratios to solve for unknown side lengths in similar triangles?
4. What does the "AAA" criterion for triangle similarity mean?
5. How does the proportionality of sides work in triangles with parallel lines?

Tip: When working with similar triangles, always check that the corresponding angles are equal and the corresponding sides are proportional.