Question

tính nhanh

\(\frac{7}{10.11}+\frac{7}{11.12}+\frac{7}{12.13}+...+\frac{7}{69.70}\)   số cuối là số 69.70 nha

các bn giúp nha

Answer 1

7/10 . 11 + 7/11.12 + 7/12.13 + ... + 7/69.70

= 7 ( 1/10.11 + 1/11.12 + 1/12.13 + ... + 1/69 . 70 )

= 7 ( 1/10 - 1/11 + 1/11 - 1/12 + 1/12 - 1/13 + ... + 1/69 - 1/70 )

= 7 ( 1/10 - 1/70 )

= 7 ( 7/70 - 1/70 )

= 7 . 6/70

= 42/70 

= 3/5 

Tk nha !!

Answer 2

=5.167

Answer 3

\(\frac{7}{10.11}+\frac{7}{11.12}+...+\frac{7}{69.70}\)

\(=\frac{7}{10}-\frac{7}{11}+\frac{7}{11}-\frac{7}{12}+...+\frac{7}{69}-\frac{7}{70}\)

\(=\frac{7}{10}-\frac{7}{70}\)

\(=\frac{7}{10}-\frac{1}{10}\)\(=\frac{6}{10}=\frac{3}{5}\)

Answer 4

7/4 (4/10x11 +4/11x12 +...+ 4/69x70)

7/4(1/10-1/11+1/11-1/12+..+1/69-1/70)

7/4(1/10-1/70)

7/4x3/35

3/20