Đáp án:
`y=19/14`
Giải thích các bước giải:
`2 . y - 1 - 2/6 - 2/12 - 2/20 - 2/30 - 2/42 = 1`
`=>2 . y - (1 + 2/6 + 2/12 + 2/20 +2/30 + 2/42 )= 1`
`=>2.y-(1+1/3+1/6+1/10+1/15+1/21)=1`
`=>2.y-[1+(1/3+1/6)+(1/10+1/15)+1/21]=1`
`=>2.y-[1+(2/6+1/6)+(3/30+2/30)+1/21]=1`
`=>2.y-[1+3/6+5/30+1/21]=1`
`=>2.y-[1+1/2+1/6+1/21]=1`
`=>2.y-[1+3/6+1/6+1/21]=1`
`=>2.y-[1+4/6+1/21]=1`
`=>2.y-[1+2/3+1/21]=1`
`=>2.y-[1+14/21+1/21]=1`
`=>2.y-[1+15/21]=1`
`=>2.y-[21/21+15/21]=1`
`=>2.y-36/21=1`
`=>2.y=1+36/21`
`=>2.y=19/7`
`=>y=19/7:2`
`=>y=19/14`
