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222-9+11+12:2*14+14 = ? ( )

Toán Lớp 7: A=1-`1/2`+`1/3`-`1/4`+`1/5`-`1/6`+-+`1/49`-`1/50` Chứng tỏ `7/12`
Tháng Sáu 28, 2022 Toán học 1 Comment 0 views

Toán Lớp 7: A=1-1/2+1/3-1/4+1/5-1/6+….+1/49-1/50
Chứng tỏ 7/12

Comments ( 1 )

  1. minhtuquynh
    minhtuquynh
    28 Tháng Sáu, 2022 at 4:21 sáng
    Reply
    #huy
    Sữa đề :  1-1/2+1/3-1/4+1/5-1/6+…+1/49-1/50
    A=(1-1/3+1/5+…+1/49)-(1/2+1/4+1/6+…+1/50)
    =(1+1/2+1/3+1/4+1/5+…+1/50)-2(1/2+1/4+1/6+…+1/50)
    =(1+1/2+1/3+1/4+…+1/50)-(1+1/2+1/3+…+1/25)
    =1/26+1/17+1/28+…+1/50<5.  1/25+10 . 1/30+10 . 1/40
    ->A<1/5+1/3+1/4=>A<1/4+1/3+1/4=5/6
    A=1/26+1/27+1/28+…+1/50
    =(1/26+1/27+…+1/30)+(1/31+1/32+1/33+…+1/40)+(1/41+1/42+…+1/50)>(1/30+1/30+…+1/30)+(1/40+1/40+…+1/40)+(1/50+1/50+1/50+…+1/50)
    ->A>5. 1/30+10 . 1/40+10. 1/50=37/60>35/60
    Vậy A>7/12
     

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222-9+11+12:2*14+14 = ? ( )

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