Đáp án:
`C<1/2`
Giải thích các bước giải:
`C=1/5+1/13+1/14+1/15+1/61+1/62+1/63`
`=>C=1/5+(1/13+1/14+1/15)+(1/61+1/62+1/63)`
Ta thấy `1/12>1/13;1/12>1/14;1/12>1/15`
`=>1/13+1/14+1/15<1/12+1/12+1/12`
`=>1/13+1/14+1/15<1/4(1)`
Ta thấy `1/60>1/61;1/60>1/62;1/60>1/63`
`=>1/61+1/62+1/63<1/60+1/60+1/60`
`=>1/61+1/62+1/63<1/20(2)`
Từ `(1)` và `(2)` ta có:
`=>1/13+1/14+1/15+1/61+1/62+1/63<1/4+1/20`
`=>1/5+1/13+1/14+1/15+1/61+1/62+1/63<1/5+1/4+1/20`
`=>1/5+1/13+1/14+1/15+1/61+1/62+1/63<9/20+1/20`
`=>1/5+1/13+1/14+1/15+1/61+1/62+1/63<1/2`
Vậy `C<1/2`.
