* * * * - * * * = * * 

1001 - 979 = 22

1000-901=99

a=17 ;b=-3,5 chac chan ban nhe

a=17

b=3,5

k mih nha  mih dang am

A = 0,5 ; B =-2 chac chan ban nhe

A=0,5

B=-2

k mih nha

mih dang ma

\(\text{Đặt }:\frac{x}{-3}=\frac{y}{5}=k\)

\(\Rightarrow x=-3k;y=5k\left(1\right)\)      và x . y = \(\frac{-5}{27}\left(2\right)\)

Thay (1) vào (2) ta có: -3k . 5k = \(\frac{-5}{27}\)

                                => -15.k² = \(\frac{-5}{27}\)

                                => k² = \(\frac{1}{81}\)

                                => k  = \(\frac{1}{9}\)và \(\frac{-1}{9}\)

Vậy k = \(\frac{1}{9}\) => x = \(\frac{-1}{3}\)và y = \(\frac{5}{9}\)

       k = \(\frac{-1}{9}\) => x = \(\frac{1}{3}\)  và y = \(\frac{-5}{9}\)

Ta có: \(\frac{1}{7}.\frac{-3}{8}+\frac{1}{7}.\frac{-13}{8}\)

\(=\frac{1}{7}.\left(\frac{-3}{8}+\frac{-13}{8}\right)\)

\(=\frac{1}{7}.\left(-2\right)\)

\(=-\frac{2}{7}\)

= \(\frac{9}{1}+\frac{8}{2}+\frac{7}{3}+\frac{6}{4}+\frac{5}{5}+\frac{4}{6}+\frac{3}{7}+\frac{2}{8}+\frac{1}{9}\)

= \(\left(9-1-1-...1\right)+\left(\frac{8}{2}+1\right)+\left(\frac{7}{3}+1\right)+...+\left(\frac{1}{9}+1\right)\)

= \(1+\frac{10}{2}+\frac{10}{3}+...+\frac{10}{9}=\frac{10}{2}+\frac{10}{3}+...+\frac{10}{9}+\frac{10}{10}\)

= .........