Giải câu 5 trang 43 Cánh Diều Toán 6 tập 2

a) $\frac{3}{16}$ : $\frac{[?]}{8}$ = $\frac{3}{4}$ 

    $\frac{[?]}{8}$ = $\frac{3}{16}$ : $\frac{3}{4}$ 

    $\frac{[?]}{8}$ = $\frac{3}{16}$ . $\frac{4}{3}$

    $\frac{[?]}{8}$ = $\frac{3 . 4}{16 . 3}$

    $\frac{[?]}{8}$ = $\frac{12}{48}$ 

    $\frac{[?]}{8}$ = $\frac{2}{8}$

    => [?] = 2 

b) $\frac{1}{25}$ : $\frac{- 3}{[?]}$ = $\frac{- 1}{15}$

    $\frac{- 3}{[?]}$ = $\frac{1}{25}$ :  $\frac{- 1}{15}$

    $\frac{- 3}{[?]}$ = $\frac{1}{25}$ . $\frac{15}{- 1}$

    $\frac{- 3}{[?]}$ = $\frac{1 . 15}{25 . (- 1)}$

    $\frac{- 3}{[?]}$ = $\frac{15}{- 25}$ 

    $\frac{- 3}{[?]}$ = $\frac{3}{- 5}$

    => [?] = 5

c) $\frac{[?]}{12}$ : $\frac{- 4}{9}$ = $\frac{- 3}{16}$

$\frac{[?]}{12}$ = $\frac{- 3}{16}$ . $\frac{- 4}{9}$

     $\frac{[?]}{12}$ = $\frac{(- 3) . (- 4)}{16 . 9}$

     $\frac{[?]}{12}$ = $\frac{12}{144}$

      $\frac{[?]}{12}$ = $\frac{1}{12}$

     => [?] = 1