Final answer:
Using the Law of Cosines, the length of side A in triangle ABC with given sides and angle is calculated to be 28 cm.
Step-by-step explanation:
To find the length of side A in triangle ABC with angle A being 120°, and sides B and C being 13 cm and 15 cm respectively, we can use the Law of Cosines. The Law of Cosines states that for any triangle ABC, the length of side a can be found using the equation:
a² = b² + c² - 2bc·cos(A)
Plugging in our values:
a² = 13² + 15² - 2·15·13·cos(120°)
Solving for a, we get:
a² = 169 + 225 - (-390)
a² = 784
a = √784
a = 28 cm
Therefore, the length of side A is 28 cm.