`P = (1.2-1)/(2!)+(2.3-1)/(3!)+(3.4-1)/(4!)+...+(99.100-1)/(100!)`
`⇒ P = (1.2)/(2!-1/2!)+(2.3)/(3!-13!)+...+(99.100-1)/(100!)`
`⇒ P =(1.2)/(2!)+ (2.3)/(3!)+ (3.4)/(4!)+...+(99.100)/(100!)-(1/(2!)+1/(3!)+...+1/(100!) )`
`⇒ P =(1+1+1/2+...+1/(98!) )-(1/(2!)+1/(3!)+...+1/(100!) )`
`⇒ P= (2-1)/(99!) - 1/(100!) <2`
`Vậy`, `P < 2`.
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