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If \(\frac{1}{a + \frac{1}{b + \frac{1}{c + \frac{1}{d + \frac{1}{e}}}}} = \frac{421}{972}\), then what is the value of \(a \times b \times c \times d \times e\)?

This question was previously asked in
CDS 2 2024 Maths Question Paper (01-Sep-2024)
The correct answer is
720

Understanding Continued Fractions

The problem asks us to find the product of the integer coefficients \(a, b, c, d, e\) in a given continued fraction.

The expression is given as:

\( \frac{1}{a + \frac{1}{b + \frac{1}{c + \frac{1}{d + \frac{1}{e}}}}}} = \frac{421}{972} \)

To find the values of \(a, b, c, d, e\), we can use the method of continued fraction expansion, which involves repeatedly taking the reciprocal and finding the integer and fractional parts.

Step-by-Step Calculation of Coefficients

First, let's invert both sides of the equation:

\( a + \frac{1}{b + \frac{1}{c + \frac{1}{d + \frac{1}{e}}}}} = \frac{972}{421} \)

Now, we perform successive divisions to find the integer parts:

  1. Divide \(972\) by \(421\):

    \( \frac{972}{421} = 2 + \frac{130}{421} \)

    Here, the integer part is \(2\). So, \(a = 2\). The remaining fraction is \(\frac{130}{421}\).

  2. Now consider the next part of the fraction, which equals \(\frac{130}{421}\). We need to find \(b\). Taking the reciprocal:

    \( b + \frac{1}{c + \frac{1}{d + \frac{1}{e}}}} = \frac{421}{130} \)

    Divide \(421\) by \(130\):

    \( \frac{421}{130} = 3 + \frac{31}{130} \)

    The integer part is \(3\). So, \(b = 3\). The remaining fraction is \(\frac{31}{130}\).

  3. Consider the next part, which equals \(\frac{31}{130}\). We need to find \(c\). Taking the reciprocal:

    \( c + \frac{1}{d + \frac{1}{e}} = \frac{130}{31} \)

    Divide \(130\) by \(31\):

    \( \frac{130}{31} = 4 + \frac{6}{31} \)

    The integer part is \(4\). So, \(c = 4\). The remaining fraction is \(\frac{6}{31}\).

  4. Consider the next part, which equals \(\frac{6}{31}\). We need to find \(d\). Taking the reciprocal:

    \( d + \frac{1}{e} = \frac{31}{6} \)

    Divide \(31\) by \(6\):

    \( \frac{31}{6} = 5 + \frac{1}{6} \)

    The integer part is \(5\). So, \(d = 5\). The remaining fraction is \(\frac{1}{6}\).

  5. Finally, we have the last term:

    \( \frac{1}{e} = \frac{1}{6} \)

    This implies \(e = 6\).

Calculating the Product \(a \times b \times c \times d \times e\)

We have found the integer values:

  • \(a = 2\)
  • \(b = 3\)
  • \(c = 4\)
  • \(d = 5\)
  • \(e = 6\)

Now, we calculate the product:

\( a \times b \times c \times d \times e = 2 \times 3 \times 4 \times 5 \times 6 \)

Performing the multiplication:

  • \(2 \times 3 = 6\)
  • \(6 \times 4 = 24\)
  • \(24 \times 5 = 120\)
  • \(120 \times 6 = 720\)

Therefore, the value of \(a \times b \times c \times d \times e\) is \(720\).

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