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

  2. It would take one machine 4 hours to complete a production order and another machine 2 hour to complete the same order. If both machines work simultaneously at their respective constant rates, the time taken to complete the same order is ________ hours.

  3. Two design consultants, P and Q, started working from 8 AM for a client. The client budgeted a total of USD 3000 for the consultants. P stopped working when the hour hand moved by 210 degrees on the clock. Q stopped working when the hour hand moved by 240 degrees. P took two tea breaks of 15 minutes each during her shift, but took no lunch break. Q took only one lunch break for 20 minutes, but no tea breaks. The market rate for consultants is USD 200 per hour and breaks are not paid. After paying the consultants, the client shall have USD_remaining in the budget.

  4. What is the value of \(1 + \frac{1}{4} + \frac{1}{{16}} + \frac{1}{{64}} + \frac{1}{{256}} + \ldots ?\)

  5. A 1.5 m tall person is standing at a distance of 3 m from a lamp post. The light from the lamp at the top of the post casts her shadow. The length of the shadow is twice her height. What is the height of the lamp post in meters?

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