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MathRevolution
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[GMAT math practice question]

Suppose \(x = \frac{a}{b} (b≠0)\). Which of following is equivalent to \(\frac{(a^2+ab)}{(a^2+b^2)}\)?

\(A. \frac{(x^2+x)}{(x^2+1)}\)

\(B. \frac{1}{(x^2+1)}\)

\(C. 1\)

\(D. x^2+1\)

\(E. x^2+x\)

Let a=2 and b=1, yielding the following:
\(x = \frac{a}{b} =\frac{2}{1} = 2\)
\(\frac{a²+ab}{a²+b²} = \frac{4+2}{4+1} =\frac{6}{5}\)

The correct answer must yield \(\frac{6}{5}\) when x=2.
Only A works:
\(\frac{x²+x}{x²+1} = \frac{4+2}{4+1} = \frac{6}{5}\)

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=>

\(\frac{(a^2+a^b)}{(a^2+b^2)}?\)

\(= { (\frac{a^2}{b^2)} + (\frac{ab}{b^2}) } / { (\frac{a^2}{b^2}) + (\frac{b^2}{b^2}) }\)

\(= { (a/b)^2 + (\frac{a}{b}) } / { (\frac{a}{b})^2 + 1 }\)

\(= \frac{(x^2+x)}{(x^2+1)}\)

Therefore, the answer is A.
Answer: A
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GMATGuruNY great explanation

Could someone please decipher what MathRevolution has done?

Surely this is not a sub-600 question
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philipssonicare
GMATGuruNY great explanation

Could someone please decipher what MathRevolution has done?

Surely this is not a sub-600 question

philipssonicare, they divided the numerator and denominator with \(b^2\).
It is valid since \(b \neq 0\).

Following is a more legible solution.

\(\begin{alignat}{2}\\
&&\cfrac{a^2+ab}{a^2+b^2} \\\\
&= & \frac{\left(\frac{a^2 + ab}{b^2}\right)}{\left(\frac{a^2 + b^2}{b^2}\right)} \\\\
&= & \frac{\left(\frac{a^2}{b^2}\right)+\left(\frac{ab}{b^2}\right)}{\left(\frac{a^2}{b^2}\right)+\left(\frac{b^2}{b^2}\right)} \\\\
&= & \cfrac{\left(\cfrac{a}{b}\right)^2+\left(\cfrac{a}{b}\right)}{\left(\cfrac{a}{b}\right)^2+1} \\\\
&= & \cfrac{x^2 + x}{x^2 + 1}\\
\end{alignat}\)

Answer is A.
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