1,914312568

Chúc bạn học tốt

=39/90

Mấy ban có cách trình bày k

~~~HD~~~

Đặt: A=5.6+6.7+..........+97.98

=> 3A=5.6(7-4)+6.7(8-5)+.......+97.98(99-96)

=> 3A=97.98.99-4.5.6=941094-120=940984

Mình không hiểu gì cả

thek ms dành cho ng` siêu hiểu ;))))

ns chung cậu tra trên google sẽ có bài lm chi tiết hơn bài lm của mk nhé

7/3.4 - 9/4.5 + 11/5.6 - 13/6.7 + 15/7.8 - 17/8.9 + 19/9.10

= 3+4/3.4 - 4+5/4.5 + 5+6/5.6 - 6+7/6.7 + 7+8/7.8 - 8+9/8.9 + 9+10/9.10

=1/3 + 1/4 - 1/4 - 1/5 + 1/5 + 1/6 -1/6 - 1/7 +1/7 +1/8 - 1/8 - 1/9 + 1/9 + 1/10

=1/3 + 1/10=13/30

\(=\frac{6+1}{12}-\frac{10-1}{20}+\frac{10+1}{30}-\frac{14-1}{42}+\frac{14+1}{56}-\frac{18-1}{72}+\frac{18+1}{90}\)

=\(\frac{6}{12}+\frac{1}{12}-\frac{10}{20}+\frac{1}{20}+\frac{10}{30}+\frac{1}{30}-\frac{14}{42}+\frac{1}{42}+\frac{14}{56}+\frac{1}{56}-\frac{18}{72}+\frac{1}{72}+\frac{18}{90}+\frac{1}{90}\)

=\(\frac{1}{2}+\frac{1}{12}-\frac{1}{2}+\frac{1}{20}+\frac{1}{3}+\frac{1}{30}-\frac{1}{3}+\frac{1}{42}+\frac{1}{4}+\frac{1}{56}-\frac{1}{4}+\frac{1}{72}+\frac{1}{5}+\frac{1}{90}\)

\(=\frac{1}{12}+\frac{1}{20}+\frac{1}{30}+\frac{1}{42}+\frac{1}{56}+\frac{1}{72}+\frac{1}{90}+\frac{1}{5}\)

\(=\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+\frac{1}{7}-\frac{1}{8}+\frac{1}{8}-\frac{1}{9}+\frac{1}{9}-\frac{1}{10}+\frac{1}{6}\)

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

\(=\frac{10}{30}-\frac{3}{30}+\frac{6}{30}=\frac{13}{30}\)

Đặt  A = 5.6 + 6.7 + 7.8 + 8.9 +...+ 97.98

 => 3A = 5.6.3 + 6.7.3 + 7.8.3 + 8.9.3 + ...+ 97.98.3

3A = 5.6.(7-4) + 6.7.(8-5) + 7.8(9-6) + 8.9.(10-7) +...+ 97.98.(99-96)

3A = 5.6.7 - 4.5.6 + 6.7.8 -5.6.7 + 7.8.9 - 6.7.8 + 8.9.10 - 7.8.9 + ...+ 97.98.99 - 96.97.98

3A = 97.98.99 - 4.5.6

A = 313 658