Đáp án + Giải thích các bước giải:
`1/(1xx2xx3)+1/(2xx3xx4)+1/(3xx4xx5)+...+1/(98xx99xx100)=x/19800`
`=>1/2(2/(1xx2xx3)+2/(2xx3xx4)+2/(3xx4xx5)+...+2/(98xx99xx100))=x/19800`
`=>1/2(1/(1xx2)-1/(2xx3)+1/(2xx3)-1/(3xx4)+1/(3xx4)-1/(4xx5)+...+1/(98xx99)-1/(99xx100))=x/19800`
`=>1/2(1/(1xx2)-1/(99xx100))=x/19800`
`=>1/2(1/2-1/9900)=x/19800`
`=>1/2(4950/9900-1/9900)=x/19800`
`=>1/2xx4949/9900=x/19800`
`=>4949/19800=x/19800`
`=>x=4949`
Vậy `x=4949`
