Answer:
∠ C ≈ 70.5°
Explanation:
using the Law of Cosines
c² = a² + b² - 2ab cosC
where c is the side opposite ∠ C and a, b the sides adjacent to ∠ C
here a = 20, b = 18 , c = 22 , then substituting into the formula
22² = 20² + 18² - ( 2 × 20 × 18 × cosC )
484 = 400 + 324 - 720cosC
484 = 724 - 720cosC ( subtract 724 from both sides )
- 240 = - 720 cosC ( divide both sides by - 720 )
= cosC , that is
cosC = 0.3333 , then
∠ C =
(0.3333) ≈ 70.5° ( to the nearest tenth )