tính nhanh nhé $\frac{3}{1.4}$ + $\frac{3}{4.7}$ +$\frac{3}{7.10}$+ …+ $\frac{3}{40.43}$ b) $\frac{1}{1.2.3}$ + $\frac{1}{2.3.4}$ +… +$\frac{1}

0 bình luận về “tính nhanh nhé $\frac{3}{1.4}$ + $\frac{3}{4.7}$ +$\frac{3}{7.10}$+ …+ $\frac{3}{40.43}$ b) $\frac{1}{1.2.3}$ + $\frac{1}{2.3.4}$ +… +$\frac{1}”

1. Giải thích các bước giải:

    `a)`

   `3/1.4 + 3/1.4 + 1/7.10 + … + 3/40.43`

   `= 1 – 1/4 + 1/4 – 1/7 + 1/7 – 1/10  + … + 1/40 – 1/43`

   `=1 – 1/43`

   `= 43/43 – 1/43`

   `=42/43`

   `b)`

   `1/(1.2.3)+ 1/(2.3.4) + …  +1/(98.99.100)`

   `= 1/2 . (1/1.2 – 1/2.3 +1/2.3 – 1/3.4 + … – 1/99.100)`

   `= 1/2 . (1/1.2 – 1/99.100)`

   `= 1/2.  (1/2 – 1/9900)`

   `=1/2. (4950/9900 – 1/9900)`

   `=1/2 . 4949/9900`

   `= 4949/19800`

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2. Giải thích các bước giải:

   a. `3/1.4+3/4.7+3/7.10+…+3/40.43`

   `=3/3(1/1.4+1/4.7+1/7.10+…+1/40.43)`

   `=1-1/4+1/4-1/7+1/7-1/10+….+1/40-1/43`

   `=1-1/43=42/43`

   b. `1/(1.2.3)+1/(2.3.4)+1/(3.4.5)+…+1/(98.99.100)`
   `=1/2(1/1.2-1/2.3+1/2.3-1/3.4+1/3.4-1/4.5+..+1/98.99-1/99.100)`
   `=1/2(1/1.2-1/99.100)`
   `=1/2. 4949/9900=4949/19800`

    

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