2a-a=1-1/2^99

a<1

Bạn có cách giải rõ ràng ko Phúc

đề có đúng ko vậy bạn

A = \(\frac{1}{2}+\frac{1}{2^2}+\frac{1}{2^3}+....+\frac{1}{2^{100}}\)

2A = \(1+\frac{1}{2}+\frac{1}{2^2}+.....+\frac{1}{2^{99}}\)

A = 2A - A = \(1-\frac{1}{2^{100}}

\(\dfrac{1}{40}+\dfrac{1}{88}+\dfrac{1}{154}+...+\dfrac{1}{y.(y+3)}=\dfrac{98}{1545}\)

\(3.(\dfrac{1}{5.8}+\dfrac{1}{8.11}+\dfrac{1}{11.14}+...+\dfrac{1}{y(y+3)})=\dfrac{98}{1545}.3\)

\(\dfrac{3}{5.8}+\dfrac{3}{8.11}+\dfrac{3}{11.14}+...+\dfrac{3}{y(y+3)}=\dfrac{98}{515}\)

\(\dfrac{1}{5}-\dfrac{1}{8}+\dfrac{1}{8}-\dfrac{1}{11}+\dfrac{1}{11}-\dfrac{1}{14}+...+\dfrac{1}{y}-\dfrac{1}{y+3}=\dfrac{98}{515}\)

\(\dfrac{1}{y+3}=\dfrac{98}{515}\)

\(y+3.1=1.515\)

\(y+3=515\)

\(y=515-3\)

\(y=512\)

Vậy y = 512

Nhớ tick cho mk nha

= 1/5.8+1/8.11+1/11.14+...+1/y(y+3)= 98/1545

=1/3.(1/5-1/8+1/8-1/11+1/11-1/14+...+1/y-1/y+3)=98/1545

=1/3.(1/5-1/y+3)=98/1545

=1/5-1/y+3=98/1545:1/3=98/515

=1/1+3=1/103

y+3=103

y=103-3

y=100

ta có A = 1+(1+2)+....+(1+2+..+100) = 1 x 100 + 2 x 99 + ...+100 x 1

\(\Rightarrow\frac{A}{100.1+99.2+...+1.100}=\frac{100.1+99.2+..+1.100}{100.1+99.2+..+100.1}=1\)  

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