Đáp án:
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`3,`
`1,4 × 15/49 - (4/5 + 2/3) ÷ 2 1/5`
`= 3/7 - (4/5 + 2/3) ÷ 11/5`
`= 3/7 - 22/15 ÷ 11/5`
`= 3/7 - 2/3`
`= 9/21 - 14/21`
`= (-5)/21`
$\\$
`4,`
`P = 2/(1×3) + 2/(3×5) + ... + 2/(95 × 97) + 2/(97 × 99)`
`-> P = (3-1)/(1×3) + (5-3)/(3×5) + ... + (97-95)/(95 × 97) + (99 - 97)/(97 × 99)`
`->P = 1 - 1/3 + 1/3 - 1/5 + ... + 1/95 - 1/97 + 1/97 - 1/99`
`-> P = 1 + (-1/3 + 1/3) + ... + (-1/97 + 1/97) - 1/99`
`-> P = 1 - 1/99`
`-> P =99/99 - 1/99`
`-> P = 98/99`
Vậy `P=98/99`