\(\frac{1}{9x10}\)\(+\frac{1}{10x11}\)\(+\frac{1}{11x12}\)\(+.....\)\(+\frac{1}{805x806}\)

\(=\frac{1}{9}\)\(-\frac{1}{10}\)\(+\frac{1}{10}\)\(-\frac{1}{11}\)\(+\frac{1}{11}\)\(-\frac{1}{12}\)\(+.....\frac{1}{805}\)\(-\frac{1}{806}\)

\(=\frac{1}{9}\)\(-\frac{1}{806}\)

\(=\frac{797}{7254}\)

Đặt:

\(X=\left(1+\dfrac{1}{9}\right)\left(1+\dfrac{1}{10}\right)\left(1+\dfrac{1}{11}\right).....\left(1+\dfrac{1}{200}\right)\)

\(X=\dfrac{10}{9}.\dfrac{11}{10}.\dfrac{12}{11}......\dfrac{201}{200}\)

\(X=\dfrac{10.11.12......201}{9.10.11......200}\)

\(X=\dfrac{201}{9}\)

\(Y=\left(1-\dfrac{1}{10}\right)\left(1-\dfrac{1}{11}\right)\left(1-\dfrac{1}{12}\right).....\left(1-\dfrac{1}{99}\right)\)

\(Y=\dfrac{9}{10}.\dfrac{10}{11}.\dfrac{11}{12}.....\dfrac{98}{99}\)

\(Y=\dfrac{9.10.11......98}{10.11.12.....99}\)

\(Y=\dfrac{9}{99}=\dfrac{1}{11}\)

Giúp mình với mai mình phải nạp cho cô giáo

=>2+8-9-x=7+15-x-x+1

=>1-x=23-2x

=>-x+2x=23+1

hay x=24

1) 9 x 10 + 10 x 11 + 11 x 12 + ....+ 2015 x 2016

=9x10x11-8x9x10+10x11x12-9x10x11+...+2015x2016x2017-2014x2015x2013

=2015x2016x2017-8x9x10

=8193537360

10 < 10 + 3

11 + 2=2 + 11

9 < 10 + 9

10 = 10 + 0

17 – 4 > 14 - 3

18 – 4 >12

15 > 15 – 1

17 + 1<17 + 2

12+ 5 > 16

16 =19 - 3

15 – 4 =10 + 1

19 – 3 >11