Đáp án:
b. \(\left\{ \begin{array}{l}
b = 40\\
a = 30
\end{array} \right.\)
Giải thích các bước giải:
\(\begin{array}{l}
a.A = \dfrac{{\left( {63 - 47} \right)\left( {63 + 47} \right)}}{{\left( {215 - 105} \right)\left( {215 + 105} \right)}}\\
= \dfrac{{16.100}}{{110.320}} = \dfrac{{16}}{{11.32}} = \dfrac{1}{{11.2}} = \dfrac{1}{{22}}\\
b.\left\{ \begin{array}{l}
a = \dfrac{3}{4}b\\
\dfrac{3}{4}b + b = 70
\end{array} \right.\\
\to \left\{ \begin{array}{l}
a = \dfrac{3}{4}b\\
\dfrac{7}{4}b = 70
\end{array} \right.\\
\to \left\{ \begin{array}{l}
b = 40\\
a = 30
\end{array} \right.
\end{array}\)
