Question

Tinh:

A=1+2+3+4+5+.......+99+100

B=1/2+1/6+1/12+1/30+...........+1/9900

Answer 1

\(A=\left(1+100\right)\times100\div2=5050\)\(A=\left(1+100\right)\times100\div2=5050\)

Answer 2

\(B=\frac{1}{2}+\frac{1}{6}+\frac{1}{12}+...+\frac{1}{9900}\)

\(\Rightarrow B=\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+...+\frac{1}{99.100}\)

\(\Rightarrow B=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+...+\frac{1}{99}-\frac{1}{100}\)

\(\Rightarrow B=1-\frac{1}{100}=\frac{99}{100}\)

Vậy..................

Answer 3

A=1+2+3+4+...+99+100

Số số hạng:(100-1):1+1=100(số)

A=(100+1)x100:2=5050

Học tốt!