Question

20^2-19^2+18^2-17^2+...+2^2-1^2

Answer 1

20^2-19^2+18^2-17^2+...+2^2-1^2 =(20^2-19^2)+(18^2-17^2)+...+(2^2-1^2) =(20+19)*(20-19)+(18+17)*(18-17)+...+(2+1)*(2-1) =20+19+18+17+...+2+1 =(20+1)*20/2 =210