`M=1/2^2 + 1/3^2 + 1/4^2 + .... + 1/20^2 `
ta thấy `1/2^2 < 1/1.2 ; 1/3^2 < 1/2.3 ; ...... ; 1/20^2 < 1/29.20`
`=> M= 1/2^2 + 1/3^2 + 1/4^2 + ..... + 1/20^2 < 1/1.2 + 1/2.3 + 1/3.4 + ..... + 1/19.20`
`= 1/1 - 1/2 + 1/2 - 1/3 + 1/3 - 1/4 +....... + 1/19 - 1/20`
`= 1 - 1/20 < 1`
mà ` M < 1 - 1/20 `
`=> M < 1`
XIN 5 SAO VS HAY NHẤT AK