S = |(-10) + (-9) + (-8) + .... + (-1)

S=(-10)+(-1)    + (-9) + (-2)   + ( -8)+(-3)   + (-7)+(-4)  +(-6)+(-5)

S=  -11          +    -11          +    -11        +     -11     + -11

S=   -11     . 5

S=       -55

tick nhé !!!!!!!!!

Cho ban 1 cong thuc ne

   Muon tinh tong cua mot day so =[ ( so hang cuoi + so hang dau) nhan so so hang ] : 2

   So so hang cua 1 tong = ( so hang cuoi - so hang dau ) : khoang cach +1

Minh nghi bai nay ban nen ap dung may tinh 

Voi 2 cong thuc nay ban se co the giai duoc

Bai lam cua minh ne 

So so hang cua day S la 

            [  ( -1 ) -(-10) ]:1 +1= 10 ( so )

S =  | { [  ( -1 ) + (-10) ] . 10 } :2 |= -55

   = |-55|

   = 55

vay S =55

Nho tick minh nhe

lấy máy tính ra mà tính

a,

=(10-1)+(10^2   -  1)+...+(10^10  - 1)

=(10 + 10^2 + 10^3 +....+ 10^10) - 10

=10^2+10^3+10^4+....+10^10

=11111111100

b,

1/21+1/28 ko bằng 2/9

\(\frac{6}{25}:\frac{21}{20}=\frac{6}{25}.\frac{20}{21}=\frac{6.20}{25.21}=\frac{2.3.4.5}{5.5.3.7}=\frac{8}{35}\)

\(\frac{40}{7}.\frac{14}{5}=\frac{40.14}{7.5}=\frac{8.5.2.7}{7.5}=\frac{16}{1}=16\)

\(\frac{17}{13}:\frac{51}{26}=\frac{17}{13}.\frac{26}{51}=\frac{17.26}{13.51}=\frac{17.2.13}{13.3.17}=\frac{2}{3}\)

Các dấu . là các dấu nhân nha

\(B=\frac{-9}{10}.\frac{5}{14}+\frac{1}{10}.\frac{-9}{2}+\frac{1}{7}.\frac{-9}{10}\)
\(B=\frac{-9}{2}.\frac{1}{14}+\frac{-9}{20}+\frac{-9}{70}\)
\(B=\frac{-9}{28}+\frac{-9}{20}+\frac{-9}{70}\)
\(B=\frac{-27}{35}+\frac{-9}{70}\)
\(B=-\frac{9}{10}\)

\(S=a^1+a^2+a^3+...+a^{13}=7\)

\(\Leftrightarrow\)\(\left(a^1+a^2+a^3\right)+\left(a^4+a^5+a^6+a^7+a^8+a^9+a^{10}+a^{11}+a^{12}\right)\)\(+a^{13}=7\)

\(\Leftrightarrow\)\(5+5+a^{13}=7\)

\(\Leftrightarrow\)\(a^{13}=7-5-5=-3\)