Bunuel wrote:
If a and b are integers, is a < 0?
(1) b^2 – a^2 < 0
(2) b^2 – a^3 > 0
\(a,b\,\,{\text{ints}}\)
\({\text{a}}\,\,\mathop {\text{ < }}\limits^? \,\,{\text{0}}\)
\(\left( 1 \right)\,\,\,{a^2} > {b^2}\,\,\,\left[ {\, \geqslant 0\,\,\,\, \Rightarrow \,\,\,{a^2} > 0\,\,\, \Rightarrow \,\,\,a \ne 0} \right]\,\,\,\,\,\,\left( * \right)\)
\(\left\{ \begin{gathered}\\
\,{\text{Take}}\,\,\left( {a,b} \right) = \left( {1,0} \right)\,\,\,\, \Rightarrow \,\,\,\left\langle {{\text{NO}}} \right\rangle \,\, \hfill \\\\
\,{\text{Take}}\,\,\left( {a,b} \right) = \left( { - 1,0} \right)\,\,\,\, \Rightarrow \,\,\,\left\langle {{\text{YES}}} \right\rangle \,\, \hfill \\ \\
\end{gathered} \right.\)
\(\left( 2 \right)\,\,{b^2} > {a^3}\,\,\,\left\{ \begin{gathered}\\
\,{\text{Take}}\,\,\left( {a,b} \right) = \left( {0,1} \right)\,\,\,\, \Rightarrow \,\,\,\left\langle {{\text{NO}}} \right\rangle \,\, \hfill \\\\
\,{\text{Take}}\,\,\left( {a,b} \right) = \left( { - 1,0} \right)\,\,\,\, \Rightarrow \,\,\,\left\langle {{\text{YES}}} \right\rangle \,\, \hfill \\ \\
\end{gathered} \right.\)
\(\left( {1 + 2} \right)\,\,\,{a^2} > {b^2} > {a^3}\,\,\,\,\,\, \Rightarrow \,\,\,\,\,\left\{ \begin{gathered}\\
{a^2} > {a^3} \hfill \\\\
\left( * \right)\,\,a \ne 0 \hfill \\ \\
\end{gathered} \right.\,\,\,\,\,\mathop \Rightarrow \limits^{:\,\,{a^2}\,\, > \,\,0} \,\,\,1 > a\,\,\,\,\,\mathop \Rightarrow \limits^{a\,\,\operatorname{int} \, \ne \,\,0} \,\,\,\,\,\left\langle {{\text{YES}}} \right\rangle\)
This solution follows the notations and rationale taught in the GMATH method.
Regards,
Fabio.