1. **Problem:** Find the missing side length in triangle 1a with sides 12 cm and 8 cm. 2. **Formula:** Use the Pythagorean theorem for right triangles or the Law of Cosines for any triangle. Since no angle is given, assume right triangle for 1a. 3. **Step:** Calculate the missing side $c$ using Pythagorean theorem: $$c=\sqrt{12^2+8^2}$$ 4. **Intermediate:** $$c=\sqrt{144+64}$$ 5. **Simplify:** $$c=\sqrt{208}$$ 6. **Final:** $$c=\sqrt{16\times 13}=4\sqrt{13}\approx 14.42\text{ cm}$$ **Answer:** The missing side length is approximately 14.42 cm.