\(\dfrac{1}{40}+\dfrac{1}{88}+\dfrac{1}{154}+...+\dfrac{1}{y.(y+3)}=\dfrac{98}{1545}\)
\(3.(\dfrac{1}{5.8}+\dfrac{1}{8.11}+\dfrac{1}{11.14}+...+\dfrac{1}{y(y+3)})=\dfrac{98}{1545}.3\)
\(\dfrac{3}{5.8}+\dfrac{3}{8.11}+\dfrac{3}{11.14}+...+\dfrac{3}{y(y+3)}=\dfrac{98}{515}\)
\(\dfrac{1}{5}-\dfrac{1}{8}+\dfrac{1}{8}-\dfrac{1}{11}+\dfrac{1}{11}-\dfrac{1}{14}+...+\dfrac{1}{y}-\dfrac{1}{y+3}=\dfrac{98}{515}\)
\(\dfrac{1}{y+3}=\dfrac{98}{515}\)
\(y+3.1=1.515\)
\(y+3=515\)
\(y=515-3\)
\(y=512\)
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
