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Question

Suppose a, b and c are three distinct natural numbers such that $a + b + c = abc$.
Consider the following statements:
1. The arithmetic mean of a, b and c is a natural number.
2. The harmonic mean of a, b and c lies between 1 and 2.
Which of the statements given above is/are correct?

The correct answer is
Both 1 and 2

The problem asks us to evaluate two statements about three distinct natural numbers $a, b, c$ that satisfy the equation $a + b + c = abc$.

Finding Distinct Natural Numbers for $a + b + c = abc$

Let the three distinct natural numbers be $a, b, c$. Without loss of generality, assume $a < b < c$. Since they are natural numbers, the smallest possible values are $a=1, b=2, c=3$.

If we set $a=1$, the equation becomes:

$1 + b + c = 1 \cdot b \cdot c$ $1 + b + c = bc$

Rearrange the terms to solve for $b$ and $c$:

$bc - b - c = 1$

Add 1 to both sides to factor the expression:

$bc - b - c + 1 = 1 + 1$ $(b-1)(c-1) = 2$

Since $a, b, c$ are distinct natural numbers and $a < b < c$, we have $1 < b < c$. This means $b-1$ and $c-1$ must be positive integers.

The integer pairs that multiply to 2 are (1, 2).

Therefore, we must have:

  • $b-1 = 1 \implies b = 2$
  • $c-1 = 2 \implies c = 3$

So, the only set of distinct natural numbers satisfying the condition is $\{1, 2, 3\}$. Let's verify: $1 + 2 + 3 = 6$ and $1 \times 2 \times 3 = 6$. The equation holds.

If $a \ge 2$, then $a \ge 2, b \ge 3, c \ge 4$. This would imply $abc \ge 2 \times 3 \times 4 = 24$. However, $a+b+c \le c+c+c = 3c$. If $a=2, b=3, c=4$, $a+b+c = 9$ and $abc = 24$. If $a, b, c$ increase, $abc$ grows much faster than $a+b+c$. If we divide the original equation by $abc$: $1/bc + 1/ac + 1/ab = 1$. If $a \ge 2$, the maximum value of the left side is $1/(2 \times 3) + 1/(2 \times 4) + 1/(3 \times 4) = 1/6 + 1/8 + 1/12 = (4+3+2)/24 = 9/24 < 1$. Thus, $a$ must be 1.

Statement 1: Arithmetic Mean (AM) Analysis

The statement says the arithmetic mean of $a, b, c$ is a natural number.

The arithmetic mean is calculated as $AM = \frac{a+b+c}{3}$.

Using the numbers $\{1, 2, 3\}$:

$AM = \frac{1+2+3}{3} = \frac{6}{3} = 2$

Since 2 is a natural number, Statement 1 is correct.

Statement 2: Harmonic Mean (HM) Analysis

The statement says the harmonic mean of $a, b, c$ lies between 1 and 2.

The harmonic mean is calculated as $HM = \frac{3}{\frac{1}{a} + \frac{1}{b} + \frac{1}{c}}$.

Using the numbers $\{1, 2, 3\}$:

$HM = \frac{3}{\frac{1}{1} + \frac{1}{2} + \frac{1}{3}}$

First, calculate the sum of the reciprocals:

$\frac{1}{1} + \frac{1}{2} + \frac{1}{3} = \frac{6}{6} + \frac{3}{6} + \frac{2}{6} = \frac{6+3+2}{6} = \frac{11}{6}$

Now, calculate the harmonic mean:

$HM = \frac{3}{\frac{11}{6}} = 3 \times \frac{6}{11} = \frac{18}{11}$

To check if $HM$ lies between 1 and 2:

  • $1 = \frac{11}{11}$
  • $2 = \frac{22}{11}$

Since $\frac{11}{11} < \frac{18}{11} < \frac{22}{11}$, we have $1 < HM < 2$.

Therefore, Statement 2 is correct.

Conclusion

Both Statement 1 and Statement 2 are correct.

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Important Questions from Integers

  1. How many three digit whole numbers are there between 75 and 405?

  2. Find the number of integers between $1$ and $150$ (inclusive) having $7$ as one of the digits but which are not divisible by $7$.

  3. Integers are listed from 700 to 1000. In how many integers is the sum of the digits 10 ?

  4. Using 2, 2, 3, 3, 3 as digits, how many distinct numbers greater than 30000 can be formed ?

  5. Consider the following statements :

    1. The sum of 5 consecutive integers can be 100.

    2 The product of three consecutive natural numbers can be equal to their sum.

    Which of the above statements is/are correct ? 

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