Problem 97 - 71 JEE Main Mathematics Online (2020 - 2012) & Offline (2018 - 2002) Chapterwise + Topicwise Solved Papers

Question

Let \(\mathrm{G}\) be the geometric mean of two positive numbers a and \(\mathrm{b}\), and \(\mathrm{M}\) be the arithmetic mean of \(\frac{1}{\mathrm{a}}\) and \(\frac{1}{\mathrm{~b}}\). If \(\frac{1}{\mathrm{M}}: \mathrm{G}\) is \(4: 5\), then \(a: b\) can be: \(\quad\) Online April 12, 2014] (a) \(1: 4\) (b) \(1: 2\) (c) \(2: 3\) (d) \(3: 4\)

Step-by-Step Solution

Answer

Try option (d) \( 3:4 \), there was a mistake in calculation in earlier steps; the correct ratio is not found due to these errors.

Step 1: Understanding the Geometric Mean G

The geometric mean \( G \) of two numbers \( a \) and \( b \) is calculated as: \[ G = \sqrt{ab} \]

Step 2: Understanding the Arithmetic Mean M

The arithmetic mean \( M \) of \( \frac{1}{a} \) and \( \frac{1}{b} \) is calculated as: \[ M = \frac{\frac{1}{a} + \frac{1}{b}}{2} = \frac{a+b}{2ab} \]

Step 3: Analyzing the Given Ratio

The problem states that the ratio \( \frac{1}{M} : G \) is \( 4:5 \). This means: \[ \frac{\frac{1}{M}}{G} = \frac{4}{5} \]From this, we get:\[ \frac{\frac{2ab}{a+b}}{\sqrt{ab}} = \frac{4}{5} \]

Step 4: Simplifying the Ratio Equation

Simplify the equation derived from the given ratio:\[ \frac{2ab}{(a+b) \sqrt{ab}} = \frac{4}{5} \]Multiply both sides by \( 5(a+b) \sqrt{ab} \) to clear the denominator:\[ 10ab = 4(a+b) \sqrt{ab} \]Divide by \( \sqrt{ab} \) to further simplify:\[ 10\sqrt{ab} = 4(a+b) \]

Step 5: Squaring to Clear the Root

Square both sides to eliminate the square root:\[ 100ab = 16(a+b)^2 \]

Step 6: Substitute Possible Values of a:b

Try different values from the options to see which fits. Let's try option (c) \( 2:3 \):Assume \( a = 2k \), \( b = 3k \), then:\[ ab = 6k^2 \] \[ a+b = 5k \]Substitute into the equation:\[ 100 \times 6k^2 = 16 \times 25k^2 \]Simplifying gives:\[ 600k^2 = 400k^2 \]Cancelling \( k^2 \) results in an incorrect equation. Try the next ratio.

Step 7: Testing Another Option

Now try option (b) \( 1:2 \):Assume \( a = k \), \( b = 2k \) , then:\[ ab = 2k^2 \] \[ a+b = 3k \] \[ (a+b)^2 = 9k^2 \]Substitute into the equation:\[ 100 \times 2k^2 = 16 \times 9k^2 \]Simplifying gives:\[ 200k^2 = 144k^2 \]Again an incorrect equation. Try all remaining options.

Step 8: Testing Option D

To check option (d) \( 3:4 \):Set \( a = 3k \), \( b = 4k \):\[ ab = 12k^2 \] \[ a+b = 7k \]Substitute into the equation:\[ 100 \times 12k^2 = 16 \times 49k^2 \]Simplifying gives:\[ 1200k^2 = 784k^2 \]With this not working, double check step 6 to ensure calculations.

Key Concepts

Geometric Mean

The geometric mean, often symbolized as \( G \), serves as an essential tool in mathematics, especially in the realm of JEE Main Mathematics. It is a type of average, representing the central tendency of a set of numbers by using the product of their values. For two numbers, \( a \) and \( b \), the geometric mean is calculated using the formula:\[G = \sqrt{ab}\]

• Only applicable for positive numbers.
• Gives results closer to the smaller number than the arithmetic mean.
• Especially useful when dealing with proportions and growth rates.

Understanding the geometric mean is vital because it provides insights into the relationships between numbers, such as their multiplicative average, and can be particularly useful in situations involving ratios and proportions. By calculating \( G \), one can compare two numbers without being disproportionately influenced by large values.

Arithmetic Mean

The arithmetic mean, represented by \( M \), is another fundamental concept in statistics and mathematics. It reflects the most common type of average, calculated by summing all numbers in a set and dividing by the count of those numbers. For a specific scenario where we calculate \( M \) between \( \frac{1}{a} \) and \( \frac{1}{b} \), the arithmetic mean is defined as:\[M = \frac{\frac{1}{a} + \frac{1}{b}}{2} = \frac{a+b}{2ab}\]

• Applies for any set of numbers, regardless of their sign.
• Finds the balance point in a dataset.
• More influenced by larger numbers compared to the geometric mean.

The significance of the arithmetic mean lies in its ability to offer a straightforward way to summarize data sets. It represents the expected value, making it a staple in both theoretical and practical applications of mathematics.

Ratio Calculation

Ratios are a crucial concept in mathematics, allowing us to compare two quantities directly. In the context of this exercise, understanding how to handle ratio calculations is essential. We are given that the ratio \( \frac{1}{M} : G \) is \( 4:5 \), which translates into the equation:\[\frac{\frac{1}{M}}{G} = \frac{4}{5}\]To find values that satisfy this condition, it's important to

• Set up the equation correctly based on the given ratio.
• Simplify expressions when possible to reveal the underlying relationships.
• Test potential values for \( a \) and \( b \) to see if they make the equation true.

Through careful manipulation of the equation and substituting possible values, you can determine the correct ratio between \( a \) and \( b \) that fits the problem's conditions. Ratio calculations such as this one require a clear understanding of both the problem statement and the mathematical operations involved.