4 + 4 + 4 + 4 + + 4 + 4 —– —– —– —— ……….. ———- ———- 2×4 4×6 6×8

0 bình luận về “4 + 4 + 4 + 4 + + 4 + 4 —– —– —– —— ……….. ———- ———- 2×4 4×6 6×8”

1. $\frac{4}{2 x 4}$ + $\frac{4}{4 x 6}$  + $\frac{4}{6 x 8}$  + $\frac{4}{8 x 10}$ +…+$\frac{4}{16x 18}$  + $\frac{4}{18 x 20}$ 

   = $\frac{2}{2}$ x ($\frac{4}{2 x 4}$ + $\frac{4}{4 x 6}$  + $\frac{4}{6 x 8}$  + $\frac{4}{8 x 10}$ +…+$\frac{4}{16x 18}$  + $\frac{4}{18 x 20}$)

   =  $\frac{4}{2}$ ($\frac{2}{2 x 4}$ + $\frac{2}{4 x 6}$  + $\frac{2}{6 x 8}$  + $\frac{2}{8 x 10}$ +…+$\frac{2}{16x 18}$  + $\frac{2}{18 x 20}$ )

   = $\frac{4}{2}$ ($\frac{1}{2}$ – $\frac{1}{4}$ + $\frac{1}{4}$ – $\frac{1}{6}$  + $\frac{1}{6}$ – $\frac{1}{8}$ + $\frac{1}{8}$ – $\frac{1}{10}$ + … + $\frac{1}{16}$ – $\frac{1}{18}$ + $\frac{1}{18}$ – $\frac{1}{20}$ 

   = $\frac{4}{2}$ ($\frac{1}{2}$ – $\frac{1}{20}$)

   = $\frac{4}{2}$ x $\frac{9}{20}$

   = $\frac{9}{10}$

   Trả lời

2. Đặt $A=\dfrac{4}{2\times4} + \dfrac{4}{4\times6} + …… + \dfrac{4}{16 \times 18} + \dfrac{4}{18 \times 20}$

   $A:4 = \dfrac{1}{2\times4} + \dfrac{1}{4\times6} + …… + \dfrac{1}{16 \times 18} + \dfrac{1}{18 \times 20}$

   $A:4 \times 2 = \dfrac{2}{2\times4} + \dfrac{2}{4\times6} + …… + \dfrac{2}{16 \times 18} + \dfrac{2}{18 \times 20}$

   $A: 2 = \dfrac{1}{2} – \dfrac{1}{4} + \dfrac{1}{4} – \dfrac{1}{6} + ….. + \dfrac{1}{16} – \dfrac{1}{18} + \dfrac{1}{18} – \dfrac{1}{20}$

   $A : 2 = \dfrac{1}{2} – \dfrac{1}{20}$

   $A : 2 = \dfrac{9}{20}$

   $A = \dfrac{9.2}{20}$

   $A = \dfrac{9}{10}$.

   Trả lời