Câu hỏi của Nguyễn Đình Nhật Nam

Question

1/3x4 + 1/4x5 + 1/5x6 + ... + 1/20x21 = x/14

Võ Ngọc Phương · Accepted Answer

$\dfrac{1}{3\times4}+\dfrac{1}{4\times5}+\dfrac{1}{5\times6}+...+\dfrac{1}{20\times21}=\dfrac{x}{14}$

$\dfrac{1}{3}-\dfrac{1}{4}+\dfrac{1}{4}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{6}+...+\dfrac{1}{20}-\dfrac{1}{21}=\dfrac{x}{14}$

$\dfrac{1}{3}-\dfrac{1}{21}=\dfrac{x}{14}$

$\dfrac{7}{21}-\dfrac{1}{21}=\dfrac{x}{14}$

$\dfrac{6}{21}=\dfrac{x}{14}$

$\Rightarrow x.21=6.14$

$x.21=84$

$x=84:21$

$x=4$

Vậy x = 4Câu trả lời: $\dfrac{1}{3\times4}+\dfrac{1}{4\times5}+\dfrac{1}{5\times6}+...+\dfrac{1}{20\times21}=\dfrac{x}{14}$

$\dfrac{1}{3}-\dfrac{1}{4}+\dfrac{1}{4}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{6}+...+\dfrac{1}{20}-\dfrac{1}{21}=\dfrac{x}{14}$

$\dfrac{1}{3}-\dfrac{1}{21}=\dfrac{x}{14}$

$\dfrac{7}{21}-\dfrac{1}{21}=\dfrac{x}{14}$

$\dfrac{6}{21}=\dfrac{x}{14}$

$\Rightarrow x.21=6.14$

$x.21=84$

$x=84:21$

$x=4$

Vậy x = 4

DSQUARED2 K9A2 · Answer

1/3x4 + 1/4x5 + 1/5x6 + .. + 1/20x21 = x/14

1/3 - 1/4 + 1/4 - 1/5 + 1/5 - 1/6 + .. + 1/20 - 1/21 = x/14

1/3 - 1/21 = x/14

7/21 - 1/21 = x/14

6/21 = x/14

x . 21 = 6 x 14

x x 21 = 84

x = 84 : 21

x = 4Câu trả lời: 1/3x4 + 1/4x5 + 1/5x6 + .. + 1/20x21 = x/14