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

Toán Lớp 5: Tính nhanh 1×2+2×3+3×4+–..+99×100

Home/ Toán Lớp 5: Tính nhanh 1×2+2×3+3×4+–..+99×100

Toán Lớp 5: Tính nhanh 1×2+2×3+3×4+–..+99×100

Tháng Mười Hai 7, 2022 Toán học 2 Comments 0 views

Toán Lớp 5: Tính nhanh
1×2+2×3+3×4+……….+99×100

Comments ( 2 )

  1. dananhthumai
    dananhthumai
    7 Tháng Mười Hai, 2022 at 3:37 sáng
    Reply
    Đặt  A = 1 × 2 + 2 × 3 + 3 × 4 + ……………. + 99 × 100
    3A = 1 × 2 × 3 + 2 × 3 × (4 – 1) + 3 × 4 × (5 – 2) + …….. + 99 × 100 × (101 – 98)
    3A = 1 × 2 × 3 + 2 × 3 × 4 – 1 × 2 × 3 + 3 × 4 × 5 – 2 × 3 × 4 + ……… + 99 × 100 × 101 – 98 × 99 × 100
    3A = 99 × 100 × 101
    3A = 999900
    A = 333300

  2. hatuanlan
    hatuanlan
    7 Tháng Mười Hai, 2022 at 3:37 sáng
    Reply
    Đặt A=1xx2+2xx3+3xx4+…+99xx100
    ->3A=3(1xx2+2xx3+3xx4+…+99xx100)
    3A=1xx2xx3+2xx3xx3+3xx4xx3+…+99xx100xx3
    3A=1xx2xx(3-0)+2xx3xx(4-1)+3xx4xx(5-2)+…+99xx100xx(101-98)
    3A=1xx2xx3+2xx3xx4-1xx2xx3+3xx4xx5-2xx3xx4+…+99xx100xx101-98xx99xx100
    3A=(1xx2xx3-1xx2xx3)+(2xx3xx4-2xx3xx4)+…+(98xx99xx100-98xx99xx100)+99xx100xx101
    3A=99xx100xx101
    3A=999900
    A=999900:3
    A=333300
    Hay 1xx2+2xx3+3xx4+…+99xx100=333300
    Vậy 1xx2+2xx3+3xx4+…+99xx100=333300

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

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