Bài 1
a) l$-3$l . $\frac{1}{2} - 3^2 . \frac{3}{2} $
= 3 . $\frac{1}{2} - 9. \frac{3}{2} $
= $\frac{3}{2} - \frac{27}{2} = -12$
b) $\frac{1}{1.2} + \frac{1}{2.3} + ... + \frac{1}{15.16}$
= $( 1 - \frac{1}{2} ) + (\frac{1}{2} - \frac{1}{3} ) +... + (\frac{1}{15}- \frac{1}{16} )$
= $1 - (\frac{1}{2} - \frac{1}{2} ) - (\frac{1}{3} + \frac{1}{3} ) -..... - (\frac{1}{15} - \frac{1}{15}) - \frac{1}{16}$
= $ 1 - \frac{1}{16} $
= $\frac{15}{16}$
Bài 2
a) $\frac{x}{-14} = \frac{2}{7}$
=> $\frac{x}{-14} = \frac{-4}{-14}$
=> $x = -4$
b) $ x + \frac{1}{3} = \frac{7}{5}$
=> $ x = \frac{7}{5} - \frac{1}{3} $
=> $ x = \frac{21}{15} - \frac{5}{15} $
=> $x = \frac{16}{15}$
