Đáp án: `A = 24/13`
Giải thích các bước giải:
`A = 4/(1xx3) + 4/(3xx5) + 4/(5xx7) + ... + 4/(11xx13)`
`1/2A = 1/2 xx 4/(1xx3) + 1/2 xx 4/(3+5) + 1/2 xx 4/(5xx7) + ... + 1/2 xx 4/(11xx13)`
`1/2A = 2/(1xx3) + 2/(3xx5) + 2/(5xx7) + ... + 2/(11xx13)`
`1/2A = 1 - 1/3 + 1/3 - 1/5 + 1/5 - 1/7 + ... + 1/11 - 1/13`
`1/2A = 1 - 1/13`
`1/2A = 12/13`
`A = 12/13 : 1/2`
`A = 12/13 xx 2/1`
`A = 24/13`