Có: `-C_2021 ^0 +C_2021 ^1 -C_2021 ^2 +....+C_2021 ^2019-C_2021 ^2020 -C_2021 ^2021 =-1-1=-2`

Mà `C_2021 ^0 +C_2021 ^1 +C_2021 ^2 +....+C_2021 ^2019 +C_2021 ^2020 +C_2021 ^2021 =2^2021`

   `=>2(C_2021 ^1 + C_2021 ^3 +C_2021 ^5 +...+C_2021 ^2017 + C_2021 ^2019 )=-2+2^2021`

 `=>C_2021 ^1 + C_2021 ^3 +...+C_2021 ^2017 + C_2021 ^2019 =-1+2^2020`

C ( y=20)

à câu c ý

Đặt A=2/3+2/6+2/12+...+2/768

=2/3(1+1/2+1/4+...+1/256)

Đặt B=1+1/2+1/4+...+1/256

=>2B=2+1+1/2+...+1/128

=>B=2-1/256=511/256

=>\(A=\dfrac{2}{3}\cdot\dfrac{511}{256}=\dfrac{511}{128\cdot3}=\dfrac{511}{384}\)

1C

6D

18D

20A

24A

29A

35D

31B

a. K mở

\(R_{tđ}=R_1+R_2=9+9=18\Omega\\ I=\dfrac{U}{R_{tđ}}=\dfrac{18}{18}=1A\)

b. K đóng

\(R_{tđ}=\dfrac{\left(R_1+R_2\right).R_3}{R_1+R_2+R_3}=\dfrac{\left(9+9\right)18}{9+9+18}=9\Omega\\ I=\dfrac{U}{R_{tđ}}=\dfrac{18}{9}=2A\)

Ta có; \(19,8:0,2\cdot44,44\cdot2\cdot13,2:0,23\)

\(=19,8\cdot5\cdot44,44\cdot26,4:0,23\)

\(=99\cdot44,44\cdot26,4:0,23\)

Ta có: \(\frac{19,8:0,2\cdot44,44\cdot2\cdot13,2:0,23}{3,3\cdot88,88:0,5\cdot6,6:0,125\cdot5}\)

\(=\frac{99\cdot44,44\cdot26,4:0,23}{3,3\cdot88,88\cdot2\cdot6,6\cdot8\cdot5}=30\cdot\frac12\cdot\frac{13.2}{6.6}\cdot\frac{1}{8\cdot5}:0,23\)

\(=15\cdot2\cdot\frac{1}{40}:\frac{23}{100}=\frac34\cdot\frac{100}{23}=3\cdot\frac{25}{23}=\frac{75}{23}\)

mắ nó mờ như tiền đồ của mình lun á b <3