IKMC 2027 - Grades 11-12 - Self-study SET 1 - Thematic Batch 3 Question 5

The sequence ($x_{n}$) is defined by $x_{1}$=a−b, $x_{2}$=c−d and $x_{n+1}$=$x_{n}$−$x_{n−1}$ for every n≥2. What is the sum of its first 2021 terms?