Câu hỏi của Dung NGuyễn

Question

Chứng miinh rằng ;3²/20.23+3²/23.26+...+3²/77.80

nguyen bao tran · Accepted Answer

tính chứ ko phải chứng minh đâu bạn?

$=3^2\left(\frac{1}{20.23}+\frac{1}{23.26}+...+\frac{1}{77.80}\right)$

$=3^2.\frac{1}{3}.\left(\frac{1}{20}-\frac{1}{23}+\frac{1}{23}-\frac{1}{26}+...+\frac{1}{77}-\frac{1}{80}\right)$

$=3\left(\frac{1}{20}-\frac{1}{80}\right)$

$=\frac{9}{80}$

Câu trả lời:

tính chứ ko phải chứng minh đâu bạn?

$=3^2\left(\frac{1}{20.23}+\frac{1}{23.26}+...+\frac{1}{77.80}\right)$

$=3^2.\frac{1}{3}.\left(\frac{1}{20}-\frac{1}{23}+\frac{1}{23}-\frac{1}{26}+...+\frac{1}{77}-\frac{1}{80}\right)$

$=3\left(\frac{1}{20}-\frac{1}{80}\right)$

$=\frac{9}{80}$

The Lonely Cancer · Answer

Đặt $A=\frac{3^2}{20\cdot23}+\frac{3^2}{23\cdot26}+\frac{....3^2}{77\cdot80}$

$A=3\left(\frac{3}{20\cdot23}+\frac{3}{23\cdot26}+....+\frac{3}{77\cdot80}\right)$

$A=3\left(\frac{1}{20}-\frac{1}{23}+\frac{1}{23}-\frac{1}{26}+...+\frac{1}{77}-\frac{1}{80}\right)$

$A=3\left(\frac{1}{20}-\frac{1}{80}\right)$

$A=3\cdot\frac{3}{80}$