Question

a) x - 20/11.13 - 20/13.15 - 20/15.17 -...- 20/53.55 = 3/11

b) 1/21 + 1/28 + 1/36+...+ 2/x(x+1) = 2/9

 

Giúp với. Khó quá!! cơ hội kiếm like :))

Answer 1

\(x-\frac{20}{11.13}-\frac{20}{23.15}-....-\frac{20}{53.55}=\frac{3}{11}\)

\(x-\left(\frac{20}{11.13}+\frac{20}{13.15}+....+\frac{20}{53.55}\right)=\frac{3}{11}\)

\(x-10\left(\frac{1}{11}-\frac{1}{13}+\frac{1}{13}-\frac{1}{15}+...+\frac{1}{53}-\frac{1}{55}\right)=\frac{3}{11}\)

\(x-10\left(\frac{1}{11}-\frac{1}{55}\right)=\frac{3}{11}\)

\(x-\frac{8}{11}=\frac{3}{11}\)

=> \(x=\frac{3}{11}+\frac{8}{11}=1\)

Answer 2

thank you nha !