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MrWhite
Let F = \(36(2x - \frac{1}{2})^3\) and G = \(16(3x - \frac{3}{4})^3\). Which of the following is true for all real numbers x?

(A) \(2F - 3G = 0\)

(B) \(3F - 2G = 0\)

(C) \(F^2 - G^2 = 0\)

(D) \(2F^2 - 3G^2 = 0\)

(E) \(3F^2 - 2G^2 = 0\)

We see that each of the options has a coefficient attached to the terms \(F\) and \(G\) and the difference between the terms is zero. Hence, the question asks us to find the coefficient that must be attached to \(F\) and \(G\) so that both the terms become equal.

\(k_1 * F = k_2 * G\)

\(k_1 * 36(2x - \frac{1}{2})^3 = k_2 * 16(3x - \frac{3}{4})^3\)

We can take a value of x such that one of the fractional parts becomes \(1\)

\(2x - \frac{1}{2} = 1\)

\(x = \frac{3}{4}\)

\(k_1 * 36((2*\frac{3}{4}) - \frac{1}{2})^3 = k_2 * 16((3*\frac{3}{4}) - \frac{3}{4})^3\)

\(k_1 * 36 = k_2 * 16*(\frac{27}{8})\)

\(\frac{k_1}{k_2} =\frac{16*27}{8* 36}\)

\(\frac{k_1}{k_2} =\frac{3}{2}\)

Hence, the ratio of the coefficients = 3:2

Option B
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\(F(0)=-9/2\\
G(0)=-27/4\\
\\
3F=-27/2\\
2G=-27/2. \)

Hence B
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­Is is just for me that the display of this question looks wrong?
Looks like a^b^c instead of a(b)^c...

Exam 3 question 16 at Official Exam Mock
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anrocha
­Is is just for me that the display of this question looks wrong?
Looks like a^b^c instead of a(b)^c...

Exam 3 question 16 at Official Exam Mock

­Due to the fractions, (2x - 1/2) and (3x - 3/4) might appear slightly higher than 36 and 16. However, they are not exponents. It looks clearer on our site.
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Yeah, I got the same question. If you solve inside the bracket for both, (4x-1)^3 can be isolated in both. Solving you get 2F/9=4G/27,
3F=2G,
3F-2G=0,
i.e B
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MrWhite
Let \(F = 36(2x - \frac{1}{2})^3\) and \(G = 16(3x - \frac{3}{4})^3\). Which of the following is true for all real numbers x?

(A) \(2F - 3G = 0\)

(B) \(3F - 2G = 0\)

(C) \(F^2 - G^2 = 0\)

(D) \(2F^2 - 3G^2 = 0\)

(E) \(3F^2 - 2G^2 = 0\)­




Look at the options. They are of the form 2F = 3G or 3F = 2G etc.
This means that we can equate the expressions and when we do, all terms will match up for the correct option.

Let's compare the x^3 terms of F and G since they are the easiest.

\(F = 36 * 2^3*x^3 + ... = 2^5 * 3^2 * x^3 + ...\)
\(G = 16 * 3^3 *x^3 + ... = 2^4*3^3*x^3+...\)

Comparing the coefficients of their x^3 terms, we see that to equate, we need 3*F = 2*G because then we will get

\(3F = 3 * 2^5 * 3^2 * x^3 + ... = 2^5*3^3*x^3 +....\) and
\(2G = 2* 2^4*3^3*x^3+...= 2^5*3^3*x^3 +....\)
which makes the coefficient of x^3 same for both

Hence answer (B)

Note that if we square F and G, more factors will be required to equate their x^3 terms so 3F^2 = 2G^2 is not possible.
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I think this approach will be much faster instead of plugging number, and easier from my perspective. Just simplify the equation first, and clearly we can tell the options with squaring variable will be eliminated. Then, we test option A & B, can reach for the correct answer with the minimum calculation.
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MrWhite
Let \(F = 36(2x - \frac{1}{2})^3\) and \(G = 16(3x - \frac{3}{4})^3\). Which of the following is true for all real numbers x?

(A) \(2F - 3G = 0\)

(B) \(3F - 2G = 0\)

(C) \(F^2 - G^2 = 0\)

(D) \(2F^2 - 3G^2 = 0\)

(E) \(3F^2 - 2G^2 = 0\)­




Given X is true for all real numbers, consider x =0

F = 36 x (-1/2)^3 = -9/2
G = 16 x (-3/4)^3 = -27/4

In what cases does this lead to a zero?

3(-9/2) – 2(-27/4) = -27/2 – (-27/2) = 0

So, 3F – 2G = 0
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Bunuel Are there any similar questions to this?
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