Đặt `n=a×(a+2)`
Ta có: `2/3+2/15+2/35+...+2/n=60/61`
`⇒2/(1×3)+2/(3×5)+2/(5×7)+...+2/n=60/61`
`⇒1/1-1/3+1/3-1/5+1/5-1/7+...+1/a-1/(a+2)=60/61`
`⇒1-1/(a+2)=60/61`
`⇒1/(a+2)=1-60/61`
`⇒1/(a+2)=1/61`
`⇒a+2=61`
`⇒a=61-2`
`⇒a=59`
Mà `n=a×(a+2)`
`⇒n=59×61=3599`
Vậy n cần tìm là `3599`