Đáp án:

(Bạn chỉ cần cho mình 1 câu trả lời hay nhất để mình lên hạng)

Giải thích các bước giải:

Bài 2 : 

$\frac{3}{5}$ + $\frac{6}{11}$ + $\frac{7}{13}$ + $\frac{2}{5}$ + $\frac{16}{11}$ + $\frac{19}{13}$

= ($\frac{3}{5}$ + $\frac{2}{5}$) + ($\frac{6}{11}$ + $\frac{16}{11}$) + ($\frac{7}{13}$ + $\frac{19}{13}$)

= 1+2+2

= 5 

$\frac{1}{1×2}$ + $\frac{1}{2×3}$ + $\frac{1}{3×4}$+ ... +$\frac{1}{9×10}$

= 1 - $\frac{1}{2}$ + $\frac{1}{2}$ - $\frac{1}{3}$+$\frac{1}{3}$ - $\frac{1}{4}$ + ... - $\frac{1}{9}$ + $\frac{1}{10}$ 

= 1 - $\frac{1}{10}$ 

= $\frac{9}{10}$ 

$\frac{1995}{1997}$ x $\frac{1990}{1993}$ x $\frac{1997}{1994}$ x $\frac{1993}{1995}$ x $\frac{997}{995}$

= $\frac{1995×1990×1997×1993×997 }{1997×1993×1994×1995×995}$

= $\frac{1990 × 997 }{1994 × 995}$ 

= 1

$\frac{5}{11}$ + $\frac{1}{4}$ + $\frac{2}{5}$ +$\frac{6}{11}$ + $\frac{3}{4}$ + $\frac{6}{25}$ +$\frac{9}{25}$

= ($\frac{5}{11}$ + $\frac{6}{11}$) + ($\frac{1}{4}$ + $\frac{3}{4}$) + ($\frac{6}{25}$ + $\frac{9}{25}$) +$\frac{2}{5}$

=  1 + 1 + ($\frac{3}{5}$ +$\frac{2}{5}$)

= 1+ 1 + 1

= 3

$\frac{1}{6}$ + $\frac{1}{12}$ + $\frac{1}{20}$ + $\frac{1}{30}$ + $\frac{1}{42}$ + $\frac{1}{56}$

= $\frac{1}{2×3}$ + $\frac{1}{3×4}$ + $\frac{1}{2×3}$ + $\frac{1}{4×5}$ + $\frac{1}{5×6}$ + $\frac{1}{6×7}$ + $\frac{1}{7×8}$

= $\frac{1}{2}$ - $\frac{1}{3}$ + $\frac{1}{3}$ - $\frac{1}{4}$ + $\frac{1}{4}$ - $\frac{1}{5}$ + $\frac{1}{5}$ - $\frac{1}{6}$ -$\frac{1}{6}$ + $\frac{1}{7}$ - $\frac{1}{7}$ + $\frac{1}{8}$

= $\frac{1}{2}$ - $\frac{1}{8}$ 

= $\frac{4}{8}$ - $\frac{3}{8}$ 

= $\frac{1}{8}$ 

Chúc bạn học tốt!!

Đáp án:

Bài 1:

a, 3990/3993;5985/5989;5985/5990;7980/7985;7980/7986

Bài 2:

1/1 × 2 + 1/2 × 3 + 1/3 × 4 ....... +1/9 × 10

= 1 - 1/2 + 1/2 - 1/3 + 1/3 + ........ + 1/9 - 1/10.

= 1 - 1/10

=9/10

5/11 + 1/4 +..........

= ( 5/11 + 6/11) + (1/4 + 3/4) + ( 6/25 + 9/25) + 2/5

= 11/11 + 4/4 + 3/5 + 2/5 

= 1 + 1 + 1

=    3