Đáp án:
`1301/1323`
Giải thích các bước giải:
`S=3/(1.2)^2+5/(2.3)^2+7/(3.4)^2+9/(4.5)^2+11/(5.6)^2+13/(6.7)^2+15/(7.9)^2 `
`=3/(1.4)+5/(4.9)+7/(9.16)+9/(16.25)+11/(25.36)+13/(36.49)+15/(49.81)`
`=1-1/4+1/4-1/9+1/9-1/16+1/16-1/25+1/25-1/36+1/36-1/49+15/(49.81)`
`=1-1/49+15/(49.81)`
`=48/49+15/(49.81)`
`=(48.81+15)/(49.81)`
`=1301/1323`