MathRevolution wrote:

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Math Revolution GMAT math practice question]

Is \(|x+y|<|x|+|y|?\)

\(1) x<0\)

\(2) y>0\)

CONCEPTUAL (THEORETICAL) TOOL: \(\left| {x + y} \right| \le \left| x \right| + \left| y \right|\)

is always valid and it is famous in Mathematics. It is the so-called

first triangle inequality.

It is useful in the GMAT to know it and, also, to know that EQUALITY is valid if, and only if, we don´t have "opposing forces" (xy<0 does not occur):

\(\left| {x + y} \right| = \left| x \right| + \left| y \right|\,\,\,\,\,\, \Leftrightarrow \,\,\,\,xy \ge 0\)

(Try some values for x and y to feel comfortable about it. In less than 3min you will realize the result is pretty evident!)

SOLUTION: (Now absolutely trivial!)

\(\left| {x + y} \right|\,\,\mathop < \limits^? \,\,\,\left| x \right| + \left| y \right|\,\,\,\,\,\,\, \Leftrightarrow \,\,\,\,\,\boxed{xy\,\,\mathop < \limits^? \,\,0}\)

\(\left( 1 \right)\,\,\,x < 0\,\,\,\,\,\left\{ \matrix{

\,{\rm{Take}}\,\,\left( {x,y} \right) = \left( { - 1,0} \right)\,\,\,\,\, \Rightarrow \,\,\,\,\left\langle {{\rm{NO}}} \right\rangle \,\, \hfill \cr

\,{\rm{Take}}\,\,\left( {x,y} \right) = \left( { - 1,1} \right)\,\,\,\, \Rightarrow \,\,\,\left\langle {{\rm{YES}}} \right\rangle \,\, \hfill \cr} \right.\)

\(\left( 2 \right)\,\,\,y > 0\,\,\,\,\,\left\{ \matrix{

\,{\rm{Take}}\,\,\left( {x,y} \right) = \left( {0,1} \right)\,\,\,\,\, \Rightarrow \,\,\,\,\left\langle {{\rm{NO}}} \right\rangle \,\, \hfill \cr

\,{\rm{Take}}\,\,\left( {x,y} \right) = \left( { - 1,1} \right)\,\,\,\, \Rightarrow \,\,\,\left\langle {{\rm{YES}}} \right\rangle \,\, \hfill \cr} \right.\)

\(\left( {1 + 2} \right)\,\,\,\,\,\left\{ \matrix{

x < 0 \hfill \cr

y > 0 \hfill \cr} \right.\,\,\,\,\,\,\,\,\, \Rightarrow \,\,\,\,\,\,\,xy < 0\,\,\,\,\,\,\, \Rightarrow \,\,\,\,\,\,\,\left\langle {{\rm{YES}}} \right\rangle \,\,\,\,\,\)

This solution follows the notations and rationale taught in the GMATH method.

Regards,

Fabio.

_________________

Fabio Skilnik :: GMATH method creator (Math for the GMAT)

Our high-level "quant" preparation starts here: https://gmath.net