Question

Answer 1

11.

\(S=\left(100-99\right)\left(100+99\right)+\left(98-97\right)\left(98+97\right)+...+\left(2-1\right)\left(2+1\right)\)

\(=1+2+...+100\)

S là tổng cấp số cộng có \(n=100\); \(u_1=1;d=1\)

\(\Rightarrow S=100.1+\dfrac{100.\left(100-1\right)}{2}.1=5050\)