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Question

For what value of k given below is \(\frac{{{{\left( {k + 2} \right)}^2}}}{{k - 3}}\) an integer?

The correct answer is

4, 8, 28

The problem asks us to find the set of values for k for which the given algebraic expression results in an integer.

The expression is given as:

$$ E(k) = \frac{{{{\left( {k + 2} \right)}^2}}}{{k - 3}} $$

To determine when this expression yields an integer, we can simplify it. Let's use a substitution to make the expression easier to handle. Let \( x = k - 3 \). This implies that \( k = x + 3 \).

Now, substitute \( k = x + 3 \) into the expression \( k + 2 \):

$$ k + 2 = (x + 3) + 2 = x + 5 $$

Substitute these back into the original expression \( E(k) \):

$$ E(k) = \frac{{(x + 5)^2}}{x} $$

Expand the numerator:

$$ E(k) = \frac{{x^2 + 2 \cdot x \cdot 5 + 5^2}}{x} = \frac{{x^2 + 10x + 25}}{x} $$

Now, divide each term in the numerator by \( x \):

$$ E(k) = \frac{{x^2}}{x} + \frac{{10x}}{x} + \frac{{25}}{x} = x + 10 + \frac{{25}}{x} $$

For \( E(k) \) to be an integer, the term \( \frac{{25}}{x} \) must also be an integer, since \( x \) (which is \( k - 3 \)) and \( 10 \) are integers (assuming \( k \) is an integer). This means that \( x \) must be a divisor of 25.

Finding Possible Values for x

The divisors of 25 are the integers that divide 25 without leaving a remainder. These are:

\( \pm 1, \pm 5, \pm 25 \)

Determining Values for k

Now, we need to find the corresponding values of \( k \) using the relationship \( k = x + 3 \):

  • If \( x = 1 \), then \( k = 1 + 3 = 4 \).
  • If \( x = -1 \), then \( k = -1 + 3 = 2 \).
  • If \( x = 5 \), then \( k = 5 + 3 = 8 \).
  • If \( x = -5 \), then \( k = -5 + 3 = -2 \).
  • If \( x = 25 \), then \( k = 25 + 3 = 28 \).
  • If \( x = -25 \), then \( k = -25 + 3 = -22 \).

So, the possible integer values for \( k \) that make the expression an integer are \( \{4, 2, 8, -2, 28, -22\} \).

Checking the Options

We need to find which of the given options contains only values from this set.

Option Values Check Against Possible k Result
1 {4, 8, 18} 4 is possible, 8 is possible, 18 is not possible. Incorrect
2 {4, 10, 16} 4 is possible, 10 is not possible, 16 is not possible. Incorrect
3 {4, 8, 28} 4 is possible, 8 is possible, 28 is possible. Correct
4 {8, 26, 28} 8 is possible, 26 is not possible, 28 is possible. Incorrect

Let's verify the values in Option 3:

  • For \( k = 4 \): \( \frac{{(4 + 2)^2}}{{4 - 3}} = \frac{{6^2}}{1} = 36 \), which is an integer.
  • For \( k = 8 \): \( \frac{{(8 + 2)^2}}{{8 - 3}} = \frac{{10^2}}{5} = \frac{100}{5} = 20 \), which is an integer.
  • For \( k = 28 \): \( \frac{{(28 + 2)^2}}{{28 - 3}} = \frac{{30^2}}{25} = \frac{900}{25} = 36 \), which is an integer.

All values in Option 3 satisfy the condition.

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Important Questions from Numerical Computation

  1. A cube of side 3 units is formed using a set of smaller cubes of side 1 unit. Find the proportion of the number of faces of the smaller cubes visible to those which are NOT visible.

  2. What is the average of all multiples of 10 from 2 to 198?

  3. Ratio of the angles of a triangle are 1 : 2 : 6. What is the smallest angle?

  4. A deposit in a bank, which pays interest on its deposits compounded daily, grows to Rs. 80,000 for 500 days and to 88,000 for 1000 days. What would be its value (in Rs.) for 1500 days?

  5. Among A, B, C and D, there is a lawyer, a doctor, a teacher and a journalist. They drink exactly one each of tea, coffee, lemonade and milk. If neither the lawyer nor the teacher drinks milk, B drinks coffee, A is the teacher and C is the doctor and drinks tea, then which of the following is FALSE?

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