\begin{tabular}{l} 9. The perimeter of a triangle is 23.4 \\ centimeters. One side is 6.7 centimeters long. \\ Another side is 6.3 centimeters long. Which \\ expression could be used to find the length of \\ the third side? \\ \\ A. \( 23.4-6.7-6.3 \) \\ B. \( 23.4-6.7+6.3 \) \\ C. \( 6.3+6.7-23.4 \) \\ (red) \\ D. \( 23.4+6.3+6.7 \) \\ \hline\end{tabular}

To find the length of the third side of the triangle, you can use the perimeter formula, which states that the perimeter equals the sum of all sides. In this case, subtracting the lengths of the two known sides from the total perimeter gives you the length of the third side. So, the correct expression is option A: \( 23.4 - 6.7 - 6.3 \). This results in \( 23.4 - 13 = 10.4 \), meaning the length of the third side is 10.4 centimeters. Triangles have fascinated mathematicians and architects for centuries, showcasing their versatility. The triangle's unique properties, including the triangle inequality theorem (where the sum of the lengths of any two sides must be greater than the length of the remaining side), are foundational in both geometry and real-world structures, like bridges and roofs. Engaging with triangles can hop you into the magical world where geometry meets engineering!