`@Kem`
`C = 1/2 × 3/4× 5/6 × .............. × 79/80`
`⇔C < 2/3 × 4/5 × 6/7 × ................ × 80/81`
`⇔C² < 1/2 × 2/3 × 3/4 × ............. × 79/80 × 80/81`
`⇔C² < 1/81`(Rút gọn lại bớt dài dòng :D)
`⇔C² < (1/9)²`
`⇔C < 1/9 < 1/7`
`⇒ C < 1/7(đpcm)`