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If \(\log_{10}(80)=p\), \(\log_{10}(45)=q\) and \(\log_{10}(216)=r\), then what is \(\log_{10}(384)\) equal to ?

This question was previously asked in
CDS 2 2026 Maths Question Paper (13-Sep-2026)
The correct answer is
\(p-q+r\)

Logarithm Simplification Strategy

The goal is to express \(\log_{10}(384)\) in terms of \(p\), \(q\), and \(r\). We first find the prime factorization of each number involved.

  • \(80 = 10 \times 8 = (2 \times 5) \times 2^3 = 2^4 \times 5\)
  • \(45 = 9 \times 5 = 3^2 \times 5\)
  • \(216 = 6^3 = (2 \times 3)^3 = 2^3 \times 3^3\)
  • \(384 = 128 \times 3 = 2^7 \times 3\)

Expressing p, q, and r

Using the properties of logarithms (\(\log(ab) = \log(a) + \log(b)\) and \(\log(a^n) = n\log(a)\)), we rewrite \(p\), \(q\), and \(r\):

  • \(p = \log_{10}(80) = \log_{10}(2^4 \times 5) = 4\log_{10}(2) + \log_{10}(5)\)
  • \(q = \log_{10}(45) = \log_{10}(3^2 \times 5) = 2\log_{10}(3) + \log_{10}(5)\)
  • \(r = \log_{10}(216) = \log_{10}(2^3 \times 3^3) = 3\log_{10}(2) + 3\log_{10}(3)\)

Target Logarithm Expression

The expression we need to find is:

\(\log_{10}(384) = \log_{10}(2^7 \times 3) = 7\log_{10}(2) + \log_{10}(3)\)

Combining p, q, and r

Let's test the combination \(p - q + r\) using the expressions derived above:

\(p - q + r = (4\log_{10}(2) + \log_{10}(5)) - (2\log_{10}(3) + \log_{10}(5)) + (3\log_{10}(2) + 3\log_{10}(3))\)

Combine like terms:

\(= (4\log_{10}(2) + 3\log_{10}(2)) + (-2\log_{10}(3) + 3\log_{10}(3)) + (\log_{10}(5) - \log_{10}(5))\)

\(= 7\log_{10}(2) + 1\log_{10}(3) + 0\)

\(= 7\log_{10}(2) + \log_{10}(3)\)

This matches the target expression for \(\log_{10}(384)\).

Conclusion

Therefore, \(\log_{10}(384)\) is equal to \(p - q + r\).

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Important Questions from Logarithms

  1. Given that $132^{0.14} = x$, $132^{0.26} = y$ and $x^z = y^2$, then the value of z is close to:

  2. For \(x \ge y > 1\), let \(\log_x\left(\frac{x}{y}\right) + \log_y\left(\frac{y}{x}\right) = k\), then the value of \(k\) can never be equal to

  3. If \(\log_ba = p\), \(\log_dc = 2p\) and \(\log_fe = 3p\), then what is \((ace)^{\frac{1}{p}}\) equal to ?

  4. What is the number of solutions of \(\log_4(x-1) = \log_2(x - 3)\)?
  5. Let \(p = \ln(x)\), \(q = \ln(x^3)\) and \(r = \ln(x^5)\), where \(x > 1\). Which of the following statements is/are correct?
    I. \(p, q\) and \(r\) are in AP.
    II. \(p, q\) and \(r\) can never be in GP.
    Select the answer using the code given below.
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