7.5:
$\dfrac{-5}{15}$ = $\dfrac{-1,2}{3,6}$
$\Rightarrow$ $\text{- 5 x 3,6 = - 1,2 x 15}$
$\dfrac{15}{-5}$ = $\dfrac{3,6}{- 1,2}$
$\dfrac{-5}{-1,2}$=$\dfrac{15 }{3,6}$
7.6: $\text{Ta có 3.81 = 9. 27. Các tỉ lệ thức lập được là:}$
$\dfrac{3}{9}$=$\dfrac{27}{81}$;$\dfrac{3}{27}$=$\dfrac{9}{81}$
$\dfrac{81}{9}$=$\dfrac{27}{3}$;$\dfrac{81}{27}$=$\dfrac{9}{3}$
7.7:
$\text{Ta có : 5 . 625 = 15. 125}$
$\Rightarrow$ $\text{Ta có 4 cặp tỉ lệ thức:}$
$\dfrac{5}{15}$=$\dfrac{125}{625}$ ; $\dfrac{625}{15}$=$\dfrac{125}{5}$
$\dfrac{5}{125}$=$\dfrac{15}{625}$ ; $\dfrac{625}{125}$=$\dfrac{15}{5}$
7.8:
a) 2,5 ; 7,5 = x : $\dfrac{3}{5}$
$\leftrightarrow$ 7,5 . x = 2,5 . $\dfrac{3}{5}$=2,5 x 0,6
$\leftrightarrow$ x = $\dfrac{2,5 .0,6}{7,5}$= 0,2
b) x. 1$\dfrac{7}{9}$ = 2$\dfrac{2}{3}$ . 0,2
$\leftrightarrow$ x . $\dfrac{16}{9}$= $\dfrac{8}{3}$ . 0,2
$\leftrightarrow$ x = $\dfrac{\dfrac{8}{3}. 0,2}{\dfrac{16}{9}}$=0,3