Đáp án+Giải thích các bước giải:
`C=1.5+2.6+3.7+...+26.30`
`=1.(0+5)+2.(1+5)+3.(2+5)+...+26.(25+5)`
`=0.1+5.1+1.2+5.2+2.3+5.3+...+25.26+5.26`
`=(0.1+1.2+2.3+...+25.26)+(5.1+5.2+5.3+...+5.26)`
`=(0.1+1.2+2.3+...+25.26)+5.(1+2+3+...+26)`
`=(0.1+1.2+2.3+...+25.26)+5.{(26+1).26}/2`
`=(0.1+1.2+2.3+...+25.26)+5.{27.26}/2`
Đặt `A=0.1+1.2+2.3+...+25.26`
`=>3A=0.1.3+1.2.3+2.3.3+...+25.26.3`
`=0+1.2.(3-0)+2.3.(4-1)+...+25.26.(27-24)`
`=1.2.(3-0)+2.3.(4-1)+...+25.26.(27-24)`
`=-0.1.2+1.2.3-1.2.3+2.3.4-...-24.25.26+25.26.27`
`=25.26.27`
`=>A={25.26.27}/3`
`=>C={25.26.27}/3+5.{27.26}/2`
`=17550/3+5.{702}/2`
`=5850+5.351`
`=5850+1755`
`=7605`
Vậy `C=7605`
