Đáp án:
`D=5/(7.9.11)+5/(9.11.13)+...+5/(59.61.63 `
`=>D/5=1/(7.9.11)+1/(9.11.13)+...+1/(59.61.63 `
`=>(4D)/5=4/(7.9.11)+4/(9.11.13)+...+4/(59.61.63 `
`=>(4D)/5=(11-7)/(7.9.11)+(13-9)/(9.11.13)+...+(63-59)/(59.61.63 `
`=>(4D)/5=1/7.9-1/9.11+1/9.11-1/11.13+...+1/59.61-1/61.63`
`=>(4D)/5=1/7.9-1/61.63`
`=>(4D)/5=1/63-1/3843`
`=>(4D)/5=20/1281`
`=>4D=20/1281*5`
`=>4D=100/1281`
`=>D=100/1281:4`
`=>D=25/1281`
`E= 4/(5.8.11 ) + 4/(8.11.14) +...+4/(17.20.23`
`=>E/4=1/(5.8.11 ) + 1/(8.11.14) +...+1/(17.20.23`
`=>(6E)/4=6/(5.8.11 ) + 6/(8.11.14) +...+6/(17.20.23`
`=>(6E)/4=(11-5)/(5.8.11 ) + (14-8)/(8.11.14) +...+(23-17)/(17.20.23`
`=>(6E)/4=1/5.8+1/8.11+1/8.11-1/11.14+...+1/17.20-1/20.23`
`=>(6E)/4=1/5.8-1/20.23`
`=>(6E)/4=1/40-1/460`
`=>(6E)/4=21/920`
`=>6E=21/920*4`
`=>6E=21/230`
`=>E=21/230:6`
`=>E=21/1380`