fskilnik
GMATH practice question (Quant Class 13)
If \(a\) and \(b\) are constants such that \(\,\,{{ax} \over {{x^2} - 1}} + {b \over {x - 1}} = {{2x - 1} \over {{x^2} - 1}}\,\,\) for every \(x > 1\), what is the value of \(a - b\) ?
(A) -2
(B) -1
(C) 0
(D) 2
(E) 4
Source:
https://www.gmath.net \(? = a - b\)
\({{ax} \over {{x^2} - 1}} + {b \over {x - 1}} = {{2x - 1} \over {{x^2} - 1}}\,\,\,\,\, \Rightarrow \,\,\,\,\,{{ax + b\left( {x + 1} \right)} \over {{x^2} - 1}} = {{2x - 1} \over {{x^2} - 1}}\,\,\,\,\, \Rightarrow \,\,\,\,\,x\left( {a + b - 2} \right) + b + 1 = 0\,\,,\,\,{\rm{for}}\,\,{\rm{all}}\,\,x > 1\,\,\,\,\left( * \right)\)
\(\left( * \right)\,\,\,\, \Rightarrow \,\,\,\,\left\{ \matrix{\\
\,b + 1 = 0\,\,\,\, \Rightarrow \,\,\,\,b = - 1 \hfill \cr \\
\,a + b - 2 = 0 \hfill \cr} \right.\,\,\,\,\,\,\, \Rightarrow \,\,\,\,\,a = 3\,\,\,\,\,\,\, \Rightarrow \,\,\,\,\,\,? = 4\)
The correct answer is therefore (E).
We follow the notations and rationale taught in the
GMATH method.
Regards,
Fabio.