retour
\begin{align*}
A &= \left(\frac 4 5 - \frac 1 {15}\right) \times \left(\frac 1 6 + \frac 1 4\right)&
\\
&=\left(\frac{12}{15}-\frac 1{15}\right)\left(\frac 2 {12} + \frac 3 {12}\right)&
\\
&=\frac{12 - 1}{15} \times \frac{2+3}{12}&
\\
&=\frac{11}{15} \times \frac 5 {12}&
\\
&=\frac{11\times 5}{15\times 12}&
\\
&=\frac{11}{3\times 12}&
\\
&=\frac{11}{36}.&
\\
B &= \frac 4 3 \times \left(\frac 1 8 - 1\right)&
\\
&= \frac 4 3 \times \left(\frac 1 8 - \frac 8 8\right)&
\\
&= \frac 4 3 \times \left(-\frac 7 8\right)&
\\
&= -\frac{4\times 7}{3\times 8}&
\\
&=-\frac 7 {3\times 2}&
\\
&=-\frac 7 6.&
\\
C&=\dfrac{\dfrac{-9} 4 \times \dfrac 5 9}{1-\dfrac 7 {12}}&
\\
&= \dfrac{-\dfrac{5}{4}}{\dfrac{12}{12}-\dfrac 7 {12}}&
\\
&=\dfrac{-\dfrac 5 4}{\dfrac 5 {12}}&
\\
&=-\dfrac 5 4 \times \dfrac {12} 5&
\\
&= -3.&
\end{align*}
retour