Mk tk cho nhé!!!

Cần chi tiết hum bẹn

có nha

\(\frac{1}{1.3}\)+\(\frac{1}{3\cdot5}\)+................+\(\frac{1}{19.21}\)

Đặt A = \(\frac{1}{1.3}\)+\(\frac{1}{3\cdot5}\)+.............+\(\frac{1}{19.21}\)

Nhân cả 2 vế của A với 2 ta có :

2A = \(\frac{2}{1.3}\)+\(\frac{2}{3.5}\)+............+\(\frac{2}{19.21}\)

2A = \(\frac{1}{1}\)-\(\frac{1}{3}\)+ \(\frac{1}{3}\) -  \(\frac{1}{5}\)+ ............ + \(\frac{1}{19}\)- \(\frac{1}{21}\)

2A = \(\frac{1}{1}\)- \(\frac{1}{21}\)

2A = \(\frac{20}{21}\)

  A = \(\frac{20}{21}\): 2 

  A = \(\frac{10}{21}\)

=1/1.3.5+1/3/5/7+1/5.7.9+......+1/17/19/21

=1/4.(5-1/1.3.5+7-3/3.5.7+.....+21-17/17/19/21

=1/4.(5/1.3.5-1/1.3.5+7/3.5.7-3/3.5.7+.....+21/17.19.21-17/17.19.21

=1/4.(1/1.3-1/3.5+1/3.5-1/5.7+.....+1/17.19-1/19.21)

=1/4.(1/3.1/21.17)

=1/4.3200/9603

= 800/9603

            Chúc bạn học tốt^^

Đặt \(A=\frac{1}{1.3.5}+\frac{1}{3.5.7}+\frac{1}{5.7.9}+...+\frac{1}{17.19.21}\)

\(\Rightarrow4A=\frac{4}{1.3.5}+\frac{4}{3.5.7}+\frac{4}{5.7.9}+...+\frac{4}{17.19.21}\)

\(=\frac{1}{1.3}-\frac{1}{3.5}+\frac{1}{3.5}-\frac{1}{5.7}+\frac{1}{5.7}-\frac{1}{7.9}+...+\frac{1}{17.19}-\frac{1}{19.21}\)

\(=\frac{1}{1.3}-\frac{1}{19.21}=\frac{44}{133}\)

\(\Rightarrow A=\frac{44}{133}\div4=\frac{11}{133}\)

A.-9/2

b.-5/7

C.5/287

D.302/105

nêu cách tính nha bạn 

sao co moi 1 cau vay ban.

a) 1-3+5-7+9-12+15-18

= (1+ 9) - (3+7) + (5+15) - (12+18) 

= 10 - 10 + 20 - 30 = -10.

b) (-2)+5-7+9-11+13-15+19-21

= 5 - 11 - 2 -2 = 5 - (11+2+2)

= 5 - 15 = -10 

Bài 1: 

a) Ta có: 1-3+5-7+9-12+15-18

=(1+9)-(3+7)+(5+15)-(12+18)

=10-10+20-30

=-10

b) Ta có: \(\left(-2\right)+5-7+9-11+13-15+19-21\)

\(=3+2+2+4-21\)

\(=5+6-21\)

=11-21=-10