1332

1375

0.68(72)

=96/97x97/98x98/99x...x999/1000

=96/1000

=12/125

x * ( x + 1 ) = 90 = 9 * 10

Do đó x = 9.

Vậy thôi mà :)

Trần Thùy Dung thiếu rồi 

Lớp 5 chưa học số âm mà Tuấn :)

\(\left(1-\frac{1}{97}\right)\times\left(1-\frac{1}{98}\right)\times...\times\left(1-\frac{1}{1000}\right)=\frac{96}{97}\times\frac{97}{98}\times...\times\frac{999}{1000}=\frac{96}{1000}=\frac{12}{125}\)

\(\frac{1}{99}-\frac{1}{99.98}-\frac{1}{98.97}-....-\frac{1}{3.2}\)

=\(\frac{1}{99}-\left(\frac{1}{2.3}+\frac{1}{3.4}+....+\frac{1}{98.99}\right)\)

=\(\frac{1}{99}-\left(\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{98}-\frac{1}{99}\right)\)

=\(\frac{1}{99}-\left(\frac{1}{2}-\frac{1}{99}\right)\)

=\(\frac{1}{99}-\frac{97}{198}\)

=\(\frac{-95}{198}\)

Ta có \(\left(1-\frac{1}{97}\right)\times\left(1-\frac{1}{98}\right)\times.....\times\left(1-\frac{1}{1000}\right).\)

\(=\frac{97-1}{97}\times\frac{98-1}{98}\times.....\times\frac{1000-1}{1000}\)

\(=\frac{96}{97}\times\frac{97}{98}\times....\times\frac{999}{1000}\) (rút gọn hết )

\(=\frac{96}{1000}\)

\(=\frac{12}{125}\)

(x+1)/99 + (x+2)/98 + (x+3)/97 + (x+4)/96 = -4

=> [(x+1)/99 +1] +[(x+2)/98+1]+[(x+3)/97+1]+[(x+4)/96+1] = 0

=>[(x+100)/99] + [(x+100)/98] +[(x+100)/97] + [(x+100)/96]=0

=>(x+100)(1/99+1/98+1/97+1/96)=0

=>x+100=0

=>x= -100